The airport in Yemen's capital Sanaa is reportedly shut after tribal fighters opposed to President Ali Abdullah Saleh clashed with government forces.
Flights were diverted to Aden after fighting broke out close to Sanaa's airport, officials said.
Meanwhile, fighters from a powerful tribal group, the Hashid, have reportedly taken control of several public buildings in Sanaa.
Witnesses say hundreds of people are fleeing the violence in the capital.
The US has ordered its non-essential embassy staff to leave the country, and advised Americans against travelling to Yemen.
At least 44 people have died in the clashes, which began on Monday after forces loyal to President Ali Abdullah 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.
The capital's main airport was reportedly forced to close after gun battles between rebels and government forces strayed too close to the facility.
The clashes pitted clansmen loyal to hardline cleric Abdul Majid al-Zindani against Republican Guard troops under the command of Mr Saleh's son Ahmed, according to AFP news agency.
Elsewhere, forces loyal to Hashid leader Sheikh Sadeq al-Ahmar seized several buildings close to his compound in the Hassaba district.
The interior ministry, national airline building and the headquarters of the state news agency Saba were all reported to have been overrun.
"Armed men stormed the Saba headquarters during the night and demanded that we leave," a Saba journalist told the AFP news agency, although Saba still appeared to be operating at some level.
Thousands of anti-Saleh protesters also marched on the city's main square, which has been the scene of regular demonstrations since February.
State TV has broadcast images of the damage done to the al-Ahmar compound by the government attack which sparked the latest violence.
A BBC correspondent in Sanaa said government forces were imposing tight security on the capital to prevent Sheikh al-Ahmar's supporters from entering.
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.