 WASHINGTON/BEIRUT The United States put Syrian President Bashar al-Assad on notice on Monday that it believes he was responsible for using chemical weapons against civilians last week in what Secretary of State John Kerry called a "moral obscenity." But while Mr Cameron "made clear there was little doubt" the attack was carried out by the Syrian regime, Mr Putin "said they did not have evidence of whether a chemical weapons attack had taken place or who was responsible," the spokesman said. Hide Caption 85 of 222 Photos: Unrest in Syria Syrian civil war in photos – A U.N. arms expert collects samples during an inspection of a suspected chemical weapons strike site in the Ghouta area outside Damascus on August 29. The agreement struck in Damascus calls for U.N. experts already in the country to begin an investigation Monday into the suspected chemical attack on rebel-held areas in the capital's eastern suburbs.  Syria agreed Sunday to a U.N. investigation into last week's alleged chemical weapons attack outside Damascus _ a deal a senior White House official dismissed as "too late to be credible," saying the United States has "very little doubt" President Bashar Assad's forces used such weapons. Media playback is unsupported on your device Media caption Russian Foreign Minister Sergei Lavrov: "They [the West] have not been able to come up with any proof but are saying at the same time that the red line has been crossed" 
 
 He said the UN Security Council, split over Syria, had not "shouldered its responsibilities". The Obama administration is now weighing how to respond in talks with U.S. allies and members of Congress, he said. Syria reached an agreement with the United Nations on Sunday to allow a U.N. team of experts to visit the site of alleged... (Associated Press) 
 
 The hardening of the U.S. position came as calls for military action grow. Syrian state media accused rebels... (Associated Press) 
 
 Syrians gather in the Marjeh square, in Damascus, Syria, Sunday, Aug. 25, 2013. Media playback is unsupported on your device Media caption William Hague: UN Security Council has not shouldered its responsibilities 
 
 But in his latest comments on the crisis, Russian Foreign Minister Sergei Lavrov said any intervention in Syria without a UN mandate would be a "grave violation of international law". In Washington, a senior administration official said the U.S. has "very little doubt" that regime forces used chemical weapons in Wednesday's attack, an assessment that was "based on the reported number of victims, reported symptoms of those who were killed or injured" as well as witness accounts and facts gathered by the U.S intelligence community. Al-Assad: It wasn't us 
 
 The Pentagon has sent four warships armed with cruise missiles to the region, and Obama will be presented with final options regarding actions against Syria in the next few days, a senior administration official said Monday. "If someone is dreaming of making Syria a puppet of the West, then this will not happen," he told the Russian newspaper Izvestiya. Speculation has been mounting that Western countries will order some kind of military response to an incident that took place a year after Obama declared the use of chemical weapons a "red line" that would require strong action. Medecins Sans Frontieres (MSF) said on Saturday that three hospitals it supports in the Damascus area had treated about 3,600 patients with "neurotoxic symptoms" on Wednesday morning, of whom 355 had died.
– Syria may have given UN inspectors the OK to access the site of last week's alleged chemical weapons attack, but inspections are not off to a great start. A UN rep confirms that one of the team's vehicles was "deliberately shot at multiple times" by unidentified snipers in Damascus; all are safe. The shooting reportedly occurred in the buffer zone between rebel- and government-controlled territory, and the team will head back to the area once it replaces its vehicle, reports the AP. The Syrian government, for its part, has already blamed "terrorists," reports CNN. More: Once they finally reach the site of last week's alleged attack, UN inspectors will spend two days taking soil, blood, urine, and tissue samples, reports the BBC. But Reuters reports that the US and its allies aren't expecting much: They believe the regime's offer to allow inspectors to the site came too late and any remaining evidence has probably been destroyed by heavy shelling. To wit, a senior White House official dismissed the visit as "too late to be credible." Assad, meanwhile, says that any American attempt at military intervention will be doomed to fail. "Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day," he said in an interview with a Russian newspaper. UN chief Ban Ki-moon warns that "every hour counts" in determining the facts of what was a "major and terrible incident," reports the Guardian. "We cannot allow impunity in what appears to be a grave crime against humanity," he said. But the UN Security Council, which is split on the issue, has already failed to shoulder its responsibilities and the UK and its allies may have to take action without its approval, Britain's foreign minister says. "Is it possible to respond to chemical weapons without complete unity on the UN Security Council?" William Hague tells the BBC. "I would argue yes it is, otherwise it might be impossible to respond to such outrages."