The US has ordered all its non-essential diplomats and family members of embassy staff to leave Yemen as fighting there escalates. At least 48 people have been killed since Monday in clashes between tribal fighters and government troops.President Ali Abdullah Saleh has again said he will not step down and leave Yemen, despite mounting protests.

He has so far refused to sign a transition deal that would see him resign in favour of a unity government.

The US State Department has also warned Americans against travelling to Yemen.

Airport clashes

"The security threat level in Yemen is extremely high due to terrorist activities and civil unrest. There is ongoing civil unrest throughout the country and large-scale protests in major cities," said the state department in a travel warning.

Yemen's Ahmar family

Sheikh Sadeq al-Ahmar is the overall leader of the Hashid tribal confederation, one of the two main tribal groupings in Yemen
His father Abdullah bin Hussein al-Ahmar - who died in 2007 - founded the Islamist Islah opposition party
Sheikh Sadeq's brother Hamid al-Ahmar is a prominent businessman and leading member of Islah. He has repeatedly called for Mr Saleh's resignation
Another brother, Sheikh Hussein bin Abdullah al-Ahmar, resigned from President Saleh's Governing People's Council on 28 February over the shootings of protesters
Like President Saleh, the Ahmars are from the Zaidi branch of Shia Islam, whereas most Yemenis are Sunni Shaf'ists

"The Department of State has ordered all eligible family members of US government employees as well as certain non-emergency personnel to depart Yemen.

"US citizens currently in Yemen should depart while commercial transportation is available."

The UK Foreign Office also advises against all travel to Yemen, as it has done since April, strongly urging any British nationals in the country to leave by commercial means.

Fighters from a powerful tribal group, the Hashid, are said to have taken control of several public buildings in the capital Sanaa after several days of fighting government troops.

Sanaa's airport was reportedly shut temporarily on Wednesday after tribal fighters opposed to President Saleh clashed with government forces.

Civil war warning

The clashes began on Monday after forces loyal to President Saleh moved against the compound of Hashid leader Sheikh Sadeq al-Ahmar.

Witnesses say hundreds of people are fleeing the violence in the capital, where residents said fighting was continuing on Thursday.

"There are still sporadic artillery hits and gunfire in Sanaa and we're unsure what's happening outside the city," said the Yemen Times' managing editor, Jeb Boone, in a telephone interview with BBC World News.

"We don't know whether tribesmen are forcing their way into the city or whether the government forces are pushing them out."

Mr Saleh refuses to stand down, despite growing international pressure.

He has warned that Yemen could descend into civil war.