(CNN) — Frederik Pleitgen. Hamdi Alkhshali and Matt Smith
Editor's note: At 6 p.m. ET, CNN will air a live "The Situation Room Special Report: Crisis in Syria." Wolf Blitzer will go in-depth with guests, analysts and reporters from around the world to examine the situation on the ground in Syria, the use of chemical weapons and the possibility of U.S. intervention.
DAMASCUS, Syria (CNN) -- U.N. experts got to inspect the site of a reported chemical attack on civilians near Syria's capital Monday as the United States accused the Syrian government of trying to cover up the attack.
The U.N. inspectors entered the town of Moadamiyet al-Sham and appeared to be examining the area accompanied by doctors, according to videos posted on social media by Syrian activists. The team had a "very productive" day and will continue its work Tuesday after examining its findings Monday evening, U.N. spokesman Farhan Haq said in New York.
Government and opposition forces have accused each other of unleashing poison gas last week in the suburban Damascus area of Ghouta. Syria's opposition said that as many as 1,300 people were killed, prompting new calls for Western powers to intervene in the country's 2-year-old civil war.
In Washington, U.S. Secretary of State John Kerry called the use of chemical weapons a "moral obscenity" that could not go unanswered, and he said Syrian actions are "not the behavior of a government that has nothing to hide."
Kerry stopped short of directly accusing President Bashar al-Assad's government of a massacre. But he said, "We know that the Syrian regime maintains custody of these chemical weapons. We know that the Syrian regime has the capacity to do this with rockets. We know that the regime has been determined to clear the opposition from those very places where the attacks took place."
Meanwhile, he said that Syria was "systemically destroying evidence" of last week's attack by continuing to shell the area and that the danger the team faced Monday "only further weakens the regime's credibility."
Monday's examination took place after unidentified snipers shot multiple times at a vehicle used by the U.N. team and after an explosion near the site inspectors planned to visit, the United Nations said. There were no reports of injuries.
U.N. officials did not say who was behind the shooting or the explosion, which witnesses said may have been caused by a mortar shell. The Syrian government accused "terrorists" of firing on the inspectors, Syrian state TV reported.
The Syrian government agreed to grant the inspectors full access on Sunday, pledging to cease all hostilities as long as the team was on the ground, the United Nations said. And an umbrella group for the Syrian opposition, the Syrian National Coalition, said rebel forces would ensure the safety of any U.N. personnel in the area.
But the government would not let U.N. inspectors approach the site for days, and the team feared that the chemical evidence may have dissipated.
'Crime against humanity'
Earlier Monday in Seoul, South Korea, U.N. Secretary-General Ban Ki-moon said the use of chemical weapons was a crime against humanity that must be punished.
"Every hour counts. We cannot afford any more delays. We have all seen the horrifying images on our television screens and through social media. Clearly, this was a major and terrible incident," Ban said.
"We owe it to the families of the victims to act."
The United States may be preparing to take on the role of the punisher, if reports that the Syrian government was behind the chemical attack are verified. The Pentagon has sent four warships armed with cruise missiles to the region, and two key foreign affairs officials expect the United States to strike Syria if the reports of chemical weapons attacks firm up.
Kerry said the Obama administration is weighing how to respond in talks with U.S. allies and members of Congress.
"Nothing today is more serious, and nothing is receiving more serious scrutiny," he said.
But as U.S. muscle has plowed the waters of the eastern Mediterranean Sea, al-Assad on Monday repeated his government's denial that his army had anything to do with the use of poison gas.
"The area of the claimed attack is in contiguity with the Syrian army positions, so how is it possible that any country would use chemical weapons in an area where its own forces are located?" he asked in an interview with Russian newspaper Izvestia.
Al-Assad accused the United States, Britain and France of exploiting the incident by trying to verify rebel allegations instead of verifying facts.
The use of a large amount of chemical weapons would cross a "red line" and threaten U.S. interests in the region, Obama has said. Washington is all but certain that al-Assad's embattled government has done so, a U.S. official said. But Obama has said he does not envision U.S. troops on the ground in Syria.
Opposition members say rockets with chemical payloads were among the ordnance government troops unleashed at the rebel stronghold of Ghouta early Wednesday. More than 1,300 people died, most of them by gas, according to opposition spokesman Khaled al-Saleh.
The opposition backed up the allegations with gruesome video of rows of dead bodies, including women and children. They had no visible wounds, and some appeared to be bloated.
The aid agency Medecins sans Frontieres said three hospitals it supports in Syria's Damascus governorate reported having received about 3,600 patients displaying neurotoxic symptoms on Wednesday morning.
But according to Syrian state-run television's depiction of events, government forces came into contact with a gas attack on Saturday in Jobar, on the edge of Damascus. Several of the soldiers were "suffocating" from exposure to gases as they entered the city, according to state TV.
"It is believed that the terrorists have used chemical weapons in the area," Syrian TV reported, citing an anonymous source. The government uses the term "terrorists" to describe rebel forces.
Broadcast video showed a room containing gas masks, gas canisters and other paraphernalia that could be used in a gas attack. The army said it uncovered the cache in a storage facility in the area.
CNN could not independently confirm the authenticity of video shown by the government or rebels.
U.S.: Al-Assad is lying
A U.S. official who was not authorized to speak to the media said the evidence pointing to government forces goes beyond images and open-source reporting from doctors and others.
"There is nothing credible to indicate that the rebels, either the Syrian National Council or even al-Nusra Front, have used chemical weapons," the official said. "Only the Assad regime is responsible for chemical weapons use."
Kerry said he had argued to Syrian Foreign Minister Walid Moallem last week that if the government had nothing to hide, "then their response should be immediate: immediate transparency, immediate access, not shelling."
"Failure to permit that, I told him, would tell its own story," he said. "Instead, for five days, the Syrian regime refused to allow the U.N. investigators access to the site of the attack that would allegedly exonerate them. Instead, it attacked the area further, shelling it and systemically destroying evidence."
But Russia, Syria's leading ally, has raised sharp objections to the possibility of any intervention in the 2-year-old conflict.
Foreign Minister Sergey Lavrov expressed concern Monday about what he characterized as the drumming up of support in the West for a military campaign against the Syrian government.
He told his U.S. counterpart Kerry by phone that Moscow is deeply concerned the U.S. military might "get involved" in the Syrian conflict, according to a statement posted to the Foreign Ministry website Monday.
At a news conference Monday, Lavrov said there's no proof yet that the Syrian government was involved in the alleged chemical weapons attack last week, and he added that the claims must be "thoroughly and professionally investigated" and submitted to the U.N. Security Council.
He said that initiatives to normalize conditions and lessen violence in Syria -- such as the peace process led by Kofi Annan, a special envoy of the United Nations and the League of Arab States -- have been undermined by Western powers that oppose and are focused on challenging the Assad government.
And Western military involvement would be futile, he argued, citing the persistent instability in Iraq and Libya after U.S.-led intervention there as examples of what could happen in Syria.
"Even if the victory of the opposition is there, the civil war will continue. The current government will become the opposition," he said.
Russia's Foreign Ministry has invoked the history of Western claims about Iraq and urged Washington to avoid "a reckless enterprise."
"All this is reminiscent of events of 10 years ago in which, using false information that the Iraqis possessed weapons of mass destruction as a pretext and bypassing the United Nations, the United States launched a reckless enterprise with consequences that everyone is well aware of," Foreign Ministry spokesman A.K. Lukashevich said in a written statement.
Al-Assad said Monday that Russia is still delivering arms to his military.