The US believes the Syrian regime’s allowing of a UN chemical weapons inspection team to probe the site of an alleged attack is too little too late, even as Moscow has urged Washington and its allies to await the team’s findings before launching military action.

Moscow on Snday night welcomed Syria’s decision to allow international experts to examine the area in a Damascus suburb where at least 100 people died in the suspected attack last week. Syrian and UN officials are working to finalize the timing of the visit.

However, a senior US official said the White House believes the Syrian government had denied the UN investigative team immediate access to the site in order to give the evidence of the attack time to degrade.

The official said the regime’s continuing shelling of the site also further corrupts any available evidence of the attack.

The Obama administration official said a belated decision to grant access to the UN team would be considered “too late to be credible.”

“At this juncture, any belated decision by the regime to grant access to the UN team would be considered too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime’s persistent shelling and other intentional actions over the last five days,” the official told the Reuters news agency.

Washington and a number of Western European countries appear to be moving toward a decision to launch military action against the embattled regime of Syrian President Bashar Assad.

On Sunday, a rebel official said opposition forces had received word that a coalition of Western states had made the decision to strike, according to Israel Radio.

Republican Sen. Bob Corker called Sunday for the US to respond in a “surgical and proportional way, something that gets their attention.” The Tennessee lawmaker said conversations with administration officials led him to believe US president Barack Obama would approach Congress for authorization to launch a strike after the summer recess next month.

Russian Foreign Ministry spokesman Alexander Lukashevich said however that all countries should wait for the results of the investigation and he encouraged other countries to “show prudence and avoid tragic mistakes” by rushing to a conclusion about the incident.

“Our American and European partners must understand what catastrophic consequences this kind of politics would have for the region, for the Arab and Islamic world as a whole,” Lukashevich said, advising the US and its allies against taking a “gamble” and using unilateral force in Syria.

Opposition figures say an early morning attack on Wednesday released chemical agents into neighborhoods east of Damascus, killing hundreds. Some rebel groups put the death toll at over 1,000 while Doctors Without Borders said Saturday that 355 people were killed and over 3,000 injured. The regime has denied carrying out the attack.

German Foreign Minister Guido Westerwelle spoke with UN disarmament chief Angela Kane about the agreement between the UN and Syria to allow experts to visit the site.

“This is an important agreement in a dramatic situation,” Westerwelle said in a statement. “I welcome that the examination will begin without delay.”

On Sunday, France and the US both said evidence suggested chemical weapons were used. Obama met with his national security team Saturday to assess the intelligence and consider a US military response, with US warships sent into the eastern Mediterranean.

Before the announcement of the UN-Syria agreement, British Prime Minister David Cameron and French President Francois Hollande discussed the situation in a telephone call Sunday morning.

“They agreed that a chemical weapons attack against the Syrian people on the scale that was emerging demanded a firm response from the international community,” a British government spokesman said on condition of anonymity because he wasn’t authorized to identify himself publicly. “This crime must not be swept under the carpet.”

In Paris, Hollande said a “body of evidence” suggests that chemical weapons were used during attacks on a Damascus suburb that killed hundreds, and that Syrian President Bashar Assad’s regime was most likely behind it. In a statement from his office, Hollande said “everything” leads France to believe the regime was behind the attack.

At the same time a White House official said there was “very little doubt” that chemical weapons had been used, with Washington basing its assessment on “the reported number of victims, reported symptoms of those who were killed or injured” and witness accounts.

Francois Heisbourg, a special adviser at the Paris-based Foundation for Strategic Research think tank, said that Syrian authorities “may have actually come to believe that there is a risk of some American strike” — and thus agreed to let the inspectors visit the suspected site.

“Whether [Syrian authorities] came to that view on their own, or whether the Russians told them to become a bit more careful, I don’t know, but it certainly looks as if they were actually starting to take the Americans seriously,” he added. “I think the Syrians are getting scared.”

Even without Western military action, rebels forces are already enjoying increased support in the wake of the chemical attack, with reports of 400 tons of Gulf-financed arms being sent from Turkey to northern based opposition fighters over the past day, Reuters reported.

The shipment would be one of the biggest arms transfers to the rebels in the two-and-a-half year-old civil war, which has claimed the lives of over 100,000 people and made over 7 million people into refugees.