The US has ordered all its non-essential diplomats and family members of embassy staff to leave Yemen as fighting there escalates. At least 44 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," the State Department said. "The Department of State has ordered all eligible family members of US government employees as well as certain non-emergency personnel to depart Yemen," the travel warning said. "US citizens currently in Yemen should depart while commercial transportation is available," it said. Fighters from a powerful tribal group, the Hashid, have reportedly taken control of several public buildings in the capital Sanaa after several days of fighting government troops. Sanaa's airport is reportedly shut after tribal fighters opposed to President Saleh clashed with government forces. Witnesses say hundreds of people are fleeing the violence in the capital. The clashes began on Monday after forces loyal to President Saleh moved against the compound of Hashid leader Sheikh Sadeq al-Ahmar. Mr Saleh refuses to stand down, despite growing international pressure. He has warned that Yemen could descend into civil war. 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. Source: BBC