More than 40 Yemenis have been killed in pitched street battles in the capital as fighting aimed at ending President Ali Abdullah Saleh's three-decade-long rule threatened to ignite civil war.
Residents were hurriedly strapping furniture, stoves, baby cots and other possessions to the roofs of cars and trucks and streaming out of Sanaa by the thousands, hoping to escape the violence that has killed more than 80 people since Monday.
The fighting, pitting the security forces of President Ali Abdullah Saleh against members of the country's most powerful Hashed tribe led by Sadiq al-Ahmar, was the bloodiest Yemen has seen since protests began in January.
The defence ministry said 28 people were killed in an explosion in an arms storage area of Sanaa earlier.
Fighters in civilian clothes roamed some districts and machinegun fire rang out sporadically.
Sporadic explosions could be heard in the capital near the protest site where thousands of people demanding Saleh to leave after nearly 33 years in power are still camped.
Black smoke from mortar fire mixed with a haze of pollution and dust that hangs over Sanaa like a shroud.
Britain said it was reducing staffing at its embassy in Yemen because of the violence. Foreign Secretary William Hague also urged British nationals to leave immediately.
Qatar also temporarily suspended the operations of its embassy in Yemen and withdrew its diplomatic staff from the country, television channel Al Jazeera said.
The US said on Wednesday it had ordered all non-essential embassy staff and family members to leave Yemen.
President Barack Obama yesterday repeated his call for Mr Saleh to step aside as pitched battles were fought between tribal groups and security forces in the capital.
'We call upon President Saleh to move immediately on his commitment to transfer power,' Mr Obama said at a joint press conference with British Prime Minister David Cameron in London.
The State Department said the threats to security were also due to activities of terrorist groups including al-Qaeda in the Arabian Peninsula (AQAP), suspected of plotting and attempting to carrying out attacks on US soil.
'The US government remains concerned about possible attacks against US citizens, facilities, businesses, and perceived US and Western interests,' said the statement.
