Yemen's President Abdrabbuh Mansour Hadi has asked the UN Security Council to back military action by "willing countries" against Houthi Shia rebels.
In a letter, he requested a resolution authorising immediate support for "the legitimate authority by all means" against the advancing rebels.
Mr Hadi is now in the southern port city of Aden after fleeing the capital Sanaa last month.
On Tuesday, the Iran-backed rebels reportedly seized two key towns.
They took full control of Sanaa in January, declaring that a five-member "presidential council" would rule the country.
Recent developments have raised fears that regional rivals Iran and Saudi Arabia could be drawn into the conflict.
On Tuesday, Saudi Arabia moved its heavy military equipment to areas near its border with Yemen, US officials were quoted as saying by Reuters news agency.
'Warplane attack'
In the letter, President Hadi asked the UN Security Council to authorise "willing countries that wish to help Yemen to provide immediate support for the legitimate authority by all means and measures to protect Yemen and deter the Houthi aggression".
"All our efforts for peaceful settlement have encountered absolute rejection by the Houthis who continue their aggression to subdue the rest of the regions out of their control".
The president earlier called on the Gulf Co-operation Council (GCC) and the Arab League to intervene.
The letter comes just hours after the rebels and soldiers loyal to Mr Hadi's ousted predecessor Ali Abdullah Saleh reportedly entered the provincial capital of Dhalea and the Red Sea port of Mukha on Tuesday.
Pro-Houthi troops also killed at least four people protesting in and around the third city of Taizz, medics said.
On Monday, Mr Hadi's foreign minister said that warplanes had targeted the palace in Aden that has been the president's base since he fled.
The Houthis: A minority Shia from the north, the group seized control of Sanaa last year and have since been expanding their control.
President Hadi: Backed by military and police loyalists, and by militia known as Popular Resistance Committees, he is trying to fight back against the rebels from his stronghold in the south.
Al-Qaeda in the Arabian Peninsula: Seen by the US as the most dangerous branch of al-Qaeda, AQAP opposes both the Houthis and forces loyal to President Hadi.
Islamic State: A Yemeni affiliate of IS has recently emerged, which seeks to eclipse AQAP.
