US withdraws diplomats from Yemen amid clashes
The US has ordered all its non-essential diplomats and family members of embassy staff to leave Yemen as fighting there escalates.
Medical sources say 72 people have died in three days of 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.
"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 Sadiq al-Ahmar.
Medical sources told the BBC that 54 people, including Ahmar supporters, women, children and neighbours were killed in fierce fighting, as well as 18 government soldiers.
More than 250 people have been injured, the sources told BBC Arabic.
Yemen's defence ministry said 28 people were killed when a munitions factory belonging to the Ahmar tribe exploded in Sanaa on Wednesday.
Mr Saleh has ordered the arrest of "rebellious" Mr Ahmar and nine of his brothers, officials said.
Meanwhile, a government spokesman said an opposition TV station, which had reportedly been off air since late on Wednesday, had been destroyed.
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, warning that Yemen could descend into civil war.
Saleh defiant
On Wednesday, US President Barack Obama said Mr Saleh should "move immediately on his commitment to transfer power".
UN Secretary General Ban Ki-moon said he was "deeply troubled" by the violence and called on all sides to find a peaceful solution.
But Mr Saleh remained defiant in a statement read by his spokesman on Wednesday.
"I will not leave power and I will not leave Yemen," the spokesman, Ahmed al-Soufi, quoted the president as saying.
The president said he was still prepared to sign a transition deal "within a national dialogue and a clear mechanism".
The deal Mr Saleh has so far refused to sign, which was presented by the Gulf Co-operation Council, calls for him to step down within a month after 33 years in office and hand over power to a unity government.
It would also give the president immunity from prosecution.
Mr Saleh has previously said he would only sign in the presence of opposition leaders.
BBC News
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