UN inspectors braved sniper fire on Monday to reach the site of a suspected deadly chemical weapons attack near Damascus, as the West weighed possible military action against Syria's regime.
A defiant President Bashar al-Assad declared that any strike by the US and its allies would be doomed to failure and key ally Moscow warned of dangerous consequences for the entire region.
A UN spokesman said unidentified snipers shot at the UN team of experts looking into allegations of a poison gas attack in a Damascus suburb last week that the Syrian opposition claimed killed hundreds of civilians.
But, after briefly pulling back, the investigators went to a field hospital in the town of Moadamiyet al-Sham where victims of the attack were still being treated, UN officials and activists said.
They took samples and interviewed medical staff during a visit which lasted almost three hours before returning to their hotel in Damascus.
The Syrian authorities accused rebels fighting Assad's forces of being behind the sniper fire, while the opposition said pro-regime militias were to blame, although both sides were said to have agreed to a temporary ceasefire.
The government had approved the UN inspection on Sunday but US officials said it was too little, too late, arguing that persistent shelling there in recent days had "corrupted" the site.
The inspection came as the West appeared to be moving closer to launching a military response over last Wednesday's attack near Damascus that shocked the world after grisly pictures emerged of dead children apparently gassed to death.
A Downing Street spokesman said British Prime Minister David Cameron was cutting short his holiday to deal with the crisis and would meet with top cabinet ministers.
British Foreign Secretary William Hague said the West could act even without full UN Security Council backing, with China and Moscow expected to boycott any resolution backing a military strike.
Washington and its allies have pointed the finger of blame at Assad's regime for the alleged attack, the latest atrocity in a conflict that has claimed the lives of more than 100,000 people since March 2011.
In the latest indictment, a US official told reporters travelling with US Defence Secretary Chuck Hagel that Washington was convinced the Syrian regime was behind the attack.