Syria expects a military attack "at any moment" and is ready to retaliate, a security official has said, hours after UN experts probing a suspected gas attack blamed on the regime quit the country.
The departure of the inspectors has opened a window for a possible US-led strike after President Barack Obama on Friday gave his clearest indication yet that a military intervention was imminent.
"We are expecting an attack at any moment. We are ready to retaliate at any moment," the security official told AFP on Saturday, asking not to be named.
The 13 UN inspectors, led by Ake Sellstrom, left their Damascus hotel in a convoy before dawn and crossed into Lebanon a few hours later, AFP reporters said.
They are due to report straight back to United Nations chief Ban Ki-moon and detail their conclusions on whether a poison gas attack actually did take place in Damascus suburbs on August 21, based on samples collected on site.
Obama's administration says it has no need to wait for the results of the probe, claiming its intelligence gives firm evidence the regime launched a chemical onslaught that killed 1429 people, including at least 426 children.
That brought a contemptuous response from Russian President Vladimir Putin, whose country is a close ally of Syria, saying claims the regime had used chemical weapons were "utter nonsense".
Putin demanded on Saturday that the US shows proof that the regime used such weapons.
"Regarding the position of our American colleagues, who affirm that government troops used ... chemical weapons, and say that they have proof, well, let them show it to the United Nations inspectors and the Security Council," he told journalists.
"If they don't show it, that means there is none."
Obama said on Friday: "We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale."
"The world has an obligation to make sure we maintain the norm against the use of chemical weapons," he said, slamming the failure of the UN Security Council to agree on action.
Obama said he was looking at a "wide range of options" but had ruled out "boots on the ground" or a "long-term campaign."
"We are looking at the possibility of a limited, narrow act," he said, while stressing no final decision had been taken on unleashing military strikes against Bashar al-Assad's regime.
France gave its backing to the US plans, saying a "strong message" should be sent to the Assad regime, but British lawmakers have voted against any involvement in military action and other close US allies said they would not sign up.
Syria has denied responsibility for the alleged incident and has pointed the finger of blame at "terrorists" - its term for rebels ranged against Assad's forces.
In Damascus, the mood was heavy with fear, and security forces were making preparations for possible air strikes, pulling soldiers back from potential targets.
The sound of bombardments in the distant suburbs could be heard, with the Syrian Observatory for Human Rights reporting shelling.
US Secretary of State John Kerry cited "multiple streams of intelligence" indicating that the Syrian government had carried out the chemical attack and that Assad himself is the "ultimate decision maker".
Kerry said failure to act would not only erode the nearly century-old norm against the use of chemical weapons, but would embolden Syrian allies Iran and Hezbollah.
While Germany and Canada ruled out joining any military strikes, French President Francois Hollande - whose country was a strident opponent of the US-led war on Iraq - said the British decision would not affect his government's stance.
Hollande said he and Obama "agreed that the international community cannot tolerate the use of chemical weapons, that it should hold the Syrian regime accountable for it and send a strong message."
Turkey, Syria's neighbour, went further still, demanding not just surgical strikes to send a message about chemical weapons but a sustained campaign to topple the regime.
Gruesome pictures of some of the reported victims of the attacks, including children, have shocked the world and piled on the pressure for a response that could draw a reluctant West into the vicious Syrian civil war.
But Russia and Iran, and even some US allies, have warned against any intervention, saying it risks sparking a wider conflict.
