SANAA (AFP) - Defiant Shi'ite 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 UN 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.

Abdul Malik al-Huthi, leader of the militia that bears the name of his late father, was defiant, warning that "all options" were open against Hadi, whom he accused of supporting the "fragmentation" of the country.

He also said his movement was ready to stand up to any measures the Security Council might adopt.

Violence has escalated in the capital, raising fears that Hadi, a key US ally in its fight against Al-Qaeda, will fall and the country descend into chaos.

Information Minister Nadia Sakkaf said the militia had launched an attack on Hadi's residence in western Sanaa, after witnesses reported clashes in the area.

Hadi was earlier reported to have been in the building meeting with advisers and security officials.

"The Yemeni President is under attack by militiamen who want to overthrow the regime," Sakkaf said on Twitter.

Witnesses said the fighting outside the residence appeared to have subsided after two soldiers were killed.

A military official told AFP the militiamen had also seized the presidential palace in southern Sanaa, where Hadi's offices are located, and were "looting its arms depots."

Prominent Huthi member Ali al-Bukhaiti said on Facebook that the fighters had "taken control of the presidential complex".

'READY TO FACE CONSEQUENCES'

In New York, the Security Council backed Hadi as "the legitimate authority" and said "all parties and political actors in Yemen must stand with" by the government "to keep the country on track to stability and security."

For her part, EU foreign policy chief Federica Mogherini said "all parties must step back immediately from conflict."

But Huthi said "all options are open in this action," and that "no one, the President or anyone else, will be above our measures if they stand to implement a conspiracy against this country."

In a long televised address, he also warned the UN Security Council that "you will not benefit from any measures you wish to take" against the Huthis.

"We are ready to face the consequences, regardless of what they are."

While calling for a full ceasefire and a return to dialogue, the council did not threaten any sanctions.

In November, it slapped sanctions on two of the militia's commanders, and on ousted president Ali Abdullah Saleh, but not on Huthi.

The fresh unrest shattered a ceasefire agreed after a bloody day Monday that saw the Huthis, who overran Sanaa in September, tighten their grip on the capital.

Militiamen and troops fought pitched battles near the presidential palace and in other parts of Sanaa, leaving at least nine people dead and 67 wounded.

The militia seized an army base overlooking the presidential palace, took control of state media and opened fire on a convoy carrying Prime Minister Khalid Bahah from Hadi's residence.

Bahah escaped to his residence, where he has lived since taking office in October, and it was surrounded by the Huthis late on Monday.

Tensions have been running high in Sanaa since Saturday, when the Huthis abducted Hadi's chief of staff, Ahmed Awad Mubarak, in an apparent move to extract changes to a draft constitution that he is overseeing.

Mubarak is in charge of a national dialogue set up after Saleh was forced from power in February 2012 following a year of bloody protests.

Saleh has been accused of backing the Huthis, and a source in the presidential guard told AFP that some troops still loyal to the former leader had supported the militia on Monday.

ALLY AGAINST AL-QAEDA

Before his kidnapping, Mubarak had been due to present a draft constitution dividing Yemen into a six-region federation, which the Huthis oppose.

The militants, who hail from the remote north and fought a decade-long war against the government, have rejected the decentralisation plan, claiming it divides the country into rich and poor regions.

Since they seized Sanaa, the Huthis have pressed their advance south of the capital, where they have met stiff resistance from Sunnis, including Al-Qaeda loyalists.

Yemen's branch of the terrorist network, Al-Qaeda in the Arabian Peninsula, is considered its most dangerous and claimed responsibility for this month's deadly attack in Paris on French satirical weekly Charlie Hebdo.

Hadi's government has been a key ally of the United States, allowing Washington to carry out repeated drone attacks on Al-Qaeda militants in its territory.