Yemen's Western-backed president, who resigned last month under pressure from Shiite militia, was out of the capital Saturday after weeks under house arrest, prompting his supporters to question U.N. proposals to fill the power vacuum.

President Abedrabbo Mansour Hadi arrived in the main southern city of Aden, where his supporters have refused to recognize the authority of the presidential council installed by the Huthi militia to replace him, an aide told AFP.

Western governments evacuated their diplomats from Yemen on Wednesday as conflict deepened in the impoverished Saudi neighbour which has long been on the front line of the war against al-Qaida.

Exactly four years after the start of Arab Spring-inspired protests that forced veteran strongman Ali Abdullah Saleh from power, his Western-backed successor is under effective house arrest, after being ousted by Shiite militia last week.

U.S. officials are holding discussions with representatives of the Shiite militia in Yemen who have forced the resignation of the country's president, a Pentagon spokesman said Tuesday.

But the discussions with the Huthi militiamen do not amount to an agreement to share intelligence on al-Qaida in Yemen, Rear Admiral John Kirby told reporters.

U.S. President Barack Obama and the new Saudi king are expected to discuss the military campaign against Islamic State group extremists and the crisis in Yemen at upcoming talks in Riyadh, the White House said Monday.

Obama's brief visit to the Saudi capital on Tuesday was mainly intended as an opportunity to meet King Salman "and to pay respects to the family and to the people of Saudi Arabia" after the death last week of long-time ruler King Abdullah, a senior official said.

Yemeni armed tribesmen arrived in Sanaa on Saturday in a bid to free top officials sieged by the powerful Shiite Huthi militia, tribal and defense sources said.

Representatives of tribes from Marib and Jawf provinces, joined by tribes from Sanaa, arrived in dozens of vehicles carrying armed men near the house of Defense Minister Mahmud Subaihi, a tribal source said.

Thousands of Yemenis took to the streets of Sanaa Saturday in the biggest protest yet against a Shiite militia takeover of the capital that has plunged the country into turmoil.

The demonstration came as regions in the formerly independent south stepped up their defiance after the Shiite militiamen, who hail from Yemen's northern highlands and are known as Huthis, tightened their grip on Sanaa.

The United States said Thursday it was assessing fast moving events in Yemen that have seen the cabinet and U.S.-backed president resign, but still supports a peaceful political transition.

"Our team is seeking confirmation of all of the reports," State Department spokeswoman Jen Psaki told reporters, adding that so far there was no move to close the U.S. embassy in the capital, Sanaa.

Defiant Shiite militiamen seized control of Yemen's presidential palace and attacked President Abdrabuh Mansur Hadi's residence Tuesday in an apparent bid to overthrow the embattled government, drawing condemnation from the U.N. Security Council.

The council backed Hadi as "the legitimate authority" and called on all parties and political actors in Yemen to stand with the government "to keep the country on track to stability and security."

Defiant Shiite militiamen seized control of Yemen's presidential palace and attacked President Abdrabuh Mansur Hadi's residence Tuesday in what officials said was a bid to overthrow his embattled government.

As the U.N. Security Council began an emergency meeting to condemn the attack and back Hadi, Secretary General Ban Ki-moon said he was "gravely concerned" and called for an immediate halt to the fighting.

Shiite militiamen surrounded Yemen's premier in his Sanaa residence after firing on his convoy during deadly clashes with the army on Monday as pressure mounted on his embattled government.

The heavily armed Huthis were in control of all three entrances to the Republican Palace, a building Prime Minister Khalid Bahah has lived in since taking office in October, a government spokesman told Agence France-Presse.