BEIRUT, Aug 29 (Reuters) : People in Damascus stocked up on supplies on Wednesday and some left homes close to potential targets as US officials described plans for multi-national strikes on Syria that could last for days.

United Nations chemical weapons experts completed a second field trip to rebel-held suburbs, looking for evidence of what — and who — caused an apparent poison gas attack that residents say killed hundreds of people a week ago.

But as UN chief Ban Ki-moon appealed for unity among world powers and sought more time for the inspectors to complete their work, Washington and its European and Middle East allies said their minds were made up and that President Bashar al-Assad must face retribution for using banned weapons against his people.

Syria’s government, supported notably by its main arms supplier Russia, cried foul. It blamed rebel “terrorists” for releasing the toxins with the help of the United States, Britain and France.

Britain pushed the other four veto-holding members of the UN Security Council at a meeting in New York to authorise military action against Assad to protect Syrian civilians — a move certain to be blocked by Russia and, probably, China.

The United States and its allies say a UN veto will not stop them.  A senior US official said strikes could last several days and would involve other armed forces.

Western armies are expected to wait until the UN experts withdraw. Their initial 14-day mandate expires in four days, and Secretary-General Ban said they need four days work.