DAMASCUS, Syria (CNN) -- U.N. inspectors appear to have reached one of the areas of an alleged chemical attack in Syria on Monday after sniper fire hit one of their vehicles.

The inspectors entered the town of Moadamiyet al-Sham and appeared to be inspecting the area with doctors, according to social media videos posted by activists.

The team had been on its way to inspect the scene of an alleged chemical weapons attack in the Damascus suburban area of Ghouta, after the Syrian government on Sunday agreed to grant the inspectors full access, the United Nations said.

The regime said its army would cease all hostilities as long as the U.N. inspectors are on the ground.

An umbrella group for the Syrian opposition, the Syrian National Coalition, also said last week that the opposition would ensure the safety of any U.N. personnel in the area.

The Syrian government accused "terrorists" of firing on the inspectors, Syrian state TV reported.

The United Nations has not said who may have been behind the shooting, which came after an explosion near the site the team planned to visit. Some witnesses said it was caused by incoming ordnance, perhaps a mortar shell.

The vehicle was "deliberately shot at multiple times by unidentified snipers in the buffer zone area," the U.N. statement said. "It has to be stressed again that all sides need to extend their cooperation so that the Team can safely carry out their important work." There were no reports of injuries.

'Crime against humanity'

The use of chemical weapons is a crime against humanity and must be punished, U.N. chief Ban Ki-moon told journalists earlier Monday in Seoul, South Korea.

Washington may be preparing to take on the role of the punisher, if reports that the Syrian government used poison gas against civilians are verified.

The team plans to inspect the site of last week's attack in Ghouta "in just a matter of hours," Ban said. The Syrian government for days would not let the team approach the site, and it fears the chemical evidence may have dissipated.

"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."

Syrian President Bashar al-Assad on Monday repeated the 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 said in an interview with Russian newspaper Izvestia.

He accused the United States, Britain and France of exploiting the incident by trying to verify rebel allegations instead of verifying facts.

A red line

The use of a large amount of chemical weapons would cross a "red line" and threaten U.S. interests in the region, President Barack Obama has said. Washington is all but certain that al-Assad's embattled government has, a U.S. official said.

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.

Obama has said he does not envision U.S. troops on the ground in Syria.

A second U.S. official said Sunday that tissue samples were collected from the scene in the hours and days after the August 21 attack "by multiple international sources" and were being analyzed in secure locations.

As U.S. muscle has plowed the waters of the eastern Mediterranean Sea, al-Assad's propaganda machine and rebel forces have pointed the finger at each other over the gassing.

But neither side denies it occurred.

Opposition accusations

Some of the rockets fired at the rebel stronghold of Ghouta by the Syrian army early Wednesday delivered chemical payloads, opposition members said.

More than 1,300 people died, most of them by gas. The opposition backed up the allegations with gruesome video of rows of dead bodies, including women and children.

They had no visible wounds. Some of them appeared to be bloated.

Doctors Without Borders said three hospitals it supports in Syria's Damascus governorate reported having received about 3,600 patients displaying neurotoxic symptoms last Wednesday morning.

Government allegations

According to Syrian state-run television's depiction of events, government forces came in contact with a gas attack on Saturday on the edge of Damascus in Jobar.

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."

A senior Obama administration official said the Syrian government's concession to U.N. inspectors was too late to be credible.

"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the U.N. -- five days ago," the official said.

The army's "persistent shelling" since the alleged chemical attack has helped to corrupt the evidence, the official, who was not authorized to speak publicly, said in a statement.

The intelligence gathered so far points to the Syrian government, he said.

Russian objections

Russian 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 John 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.

In a news conference Monday, Lavrov told reporters there's no proof yet that the Syrian government was involved in an 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.

He added that any Western military involvement would be futile. In arguing against Western intervention, Lavrov cited the persistent instability in Iraq and Libya 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.

CNN's Fred Pleitgen reported from Syria. Ben Brumfield wrote and reported from Atlanta. CNN's Hamdi Alkhshali reported from Atlanta and Jomana Karadsheh from Jordan. CNN's Boriana Milanova, Chris Lawrence, Jim Acosta, Josh Levs, Joe Sterling, Elise Labott and Jill Dougherty contributed to this report.

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