The United Nations Security Council appeared headed for a new confrontation over Syria on Wednesday after Britain said it would introduce a resolution accusing the Syrian government of a deadly chemical weapons attack last week and authorizing the use of force in that conflict, a measure that Russia was almost certain to block.

The Russians, who are the Syrian government's most powerful foreign allies, argued that it was premature to even talk about such a resolution while United Nations inspectors were on the ground in Syria investigating the allegations surrounding the Aug. 21 attack in the suburbs of the capital, Damascus. Opposition figures and rights groups have said that hundreds of civilians were killed.

That attack has galvanized Western efforts that could lead to a military strike on Syria, which is well into the third year of a brutal civil conflict that has already killed more than 100,000 people and left millions displaced.

Syria's ambassador to the United Nations, Bashar Jaafari, added a new level of complexity to the issue on Wednesday, announcing that he had submitted evidence of three previously unreported instances of chemical weapons use in Syria, which he asserted had been carried out by Syrian insurgents. Mr. Jaafari said the Syrian government had requested that the United Nations investigators expand their inquiry to include those events as well, which could lengthen their stay in the country.

Mr. Jaafari said the new instances occurred on Aug. 22, 24 and 25, and were also in the Damascus suburbs. He said Syrian soldiers were the targets. The ambassador did not explain why he was only now bringing forth the allegations, which critics were likely to view as a stalling exercise.

David Fuller's view The US and its allies who are intent on bombing Syria's military aircraft bases will have a legitimacy problem if United Nations inspectors do not conclude, beyond any doubt, that the Assad regime was responsible for the gas attacks. This may not be easy, given the delays, and UN inspectors have not been helped by Western politicians who have already offered their verdict before the investigation has been completed. Moreover, most of Assad's supporters and anyone in the Middle East who is hostile to Western interests - there are plenty of them - will believe whatever they are told by their local leaders.