Dozens of UN staff have left Syria and several countries have warned citizens to stay away from neighbouring Lebanon as regional tensions grow over a possible US-led military strike.

US President Barack Obama has said he is planning a "limited, narrow" military action to punish Syria's President Bashar al-Assad for a poison gas attack that Washington said killed 1,429 people.

Syria denies the accusation and says rebels trying to overthrow Assad were responsible.

Several UN agencies had scaled down their staffing in Syria as a precaution, a UN source told Reuters from Damascus on Saturday.

"Most of the midlevel and nonessential foreign staff left on Thursday. The heads of the various agencies have stayed behind, together with a skeleton local staff," the source added.

The United Nations has about 1,000 national and international staff working on humanitarian and relief projects in Syria, spokeswoman Alessandra Vellucci said on Friday. She said she could not comment on reports of UN staff leaving the country.

UN agencies active in Syria include refugee agency UNHCR, the UN Children's Fund (UNICEF), the World Health Organisation and the World Food Programme.

Bahrain, Kuwait, Britain and France warned their nationals not to travel to Lebanon, whose divided communities have been pulled into the increasingly sectarian conflict next door.

Austria told its citizens to contact its embassy in Lebanon before travelling there.

A senior security source in Lebanon said 14,000 people had left the country on Thursday alone, mostly Europeans.

Bahrain's foreign ministry said its advice was prompted by growing concerns of the impact of the Syrian crisis on Lebanon, news agency BNA reported late on Friday. Kuwait's embassy in Lebanon told its citizens to leave the country due to "security uncertainty", news agency KUNA said.

Britain advised against all but essential travel to Lebanon on Friday, citing the recent violence there and wider regional tensions. It also said there may be an increased risk of anti-Western sentiment linked to the possibility of military action. France issued similar advice on Thursday.

At least 100,000 people have been killed and two million driven abroad by Syria's conflict.-Reuters