US President Barack Obama was reviewing Saturday a possible response to an alleged chemical "massacre" in Syria, as a top UN official was pressing Damascus for an investigation into the charges.
Obama is under mounting pressure to act following reports of a massive chemical attack near Damascus on Wednesday that opposition groups say was carried out by President Bashar al-Assad's forces and had killed as many as 1,300 people.
The Syrian government has strongly denied those allegations, but has yet to accede to demands that UN inspectors already in the country be allowed to visit the sites of the alleged attacks.
As Defence Secretary Chuck Hagel suggested the US Navy was moving forces closer to Syria, Obama met his top national security advisors to discuss the response to the alleged use of chemical weapons, a White House official said.
"We have a range of options available, and we are going to act very deliberately so that we're making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria," the official said.
On Friday, Hagel said his department "has a responsibility to provide the president with options for all contingencies," which includes "positioning our forces...to be able to carry out different options -- whatever the president might choose."
But he declined to provide any details on the deployment of those forces, as the Obama administration reportedly contemplated cruise missile strikes against Assad's forces.
The New York Times cited a senior administration official as saying Washington was looking at NATO's air war over Kosovo in 1999 as a blueprint for strikes on Syria without a UN mandate.
Obama had repeatedly warned that the use of chemical weapons by Syrian forces was a "red line" that could bring about a more strident Western response to the 29-month-old civil war.
But he has also said Washington must be wary of costly and difficult foreign interventions.
On Thursday, French Foreign Minister Laurent Fabius said that if the massacre were confirmed, "there must be a reaction, a reaction that could take the form of a reaction with force."
On Saturday, during a visit to the West Bank, he said the regime carried out a "chemical massacre" and that "the Bashar regime is responsible."
That followed similar accusations from his British Foreign Secretary William Hague, who said Syria carried out a "large scale" chemical attack and insisted that Damascus give UN inspectors access to the suspect sites.
But Damascus ally Iran blamed the rebels and warned the West against any military intervention.
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"There is proof terrorist groups carried out this action," foreign ministry spokesman Abbas Araqchi said, without giving any details.
Warning against any Western military intervention in the conflict, Araqchi said "there is no international authorisation for" such action.
"We warn against any actions or statements that could create more tension in the region. I hope that White House officials show enough wisdom not to enter into such dangerous tumult."
Meanwhile, UN Under Secretary General Angela Kane was in Damascus, tasked by Secretary General Ban Ki-moon with establishing the terms of an inquiry.
Ban is determined to "conduct a thorough, impartial and prompt investigation" into the chemical attack claims, his spokesman said.
On Friday, Syria's main opposition National Coalition pledged to guarantee the safety of the inspectors but warned that the "clock is ticking" before alleged evidence vanishes.
Coalition chief Ahmad al-Jarba was scheduled to hold a press conference on the issue in Istanbul later on Saturday.
Syria has yet to say if it will let the UN experts -- on the ground since August 18 to probe three other sites -- to inspect the latest allegations.
Russia urged Damascus to cooperate with the UN but dismissed calls for use of force against its ally.
In statements published on Saturday, German Chancellor Angela Merkel accused Russia and China of having blocked a UN text demanding the inspectors be given unfettered access.
One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".
The United Nations says more than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, while millions more have fled the country or been internally displaced.
The violence continued Saturday, with a watchdog accusing the regime of striking by air several rebel positions, including in Jobar, and reporting that insurgents seized a strategic town in the northwest.
State television said an army unit was surrounding a "sector of Jobar where terrorists used chemical weapons," adding that soldiers who tried to enter the neighbourhood had "suffocated."
Rebels have "resorted to chemical weapons after the successes of the Syrian army in recent days," the television charged.