Yemeni political factions resumed UN-brokered talks on Monday with the Shia militia that grabbed power last week, but one group walked out underlining the complexity of the crisis.

Yemen, a deeply tribal country awash with weapons, has been riven by crisis since veteran strongman Ali Abdullah Saleh was forced from power in 2012 following a bloody year-long uprising against his rule.

The country is also battling an Al-Qaeda insurgency and facing a separatist movement in the formerly independent south.

UN envoy Jamal Benomar has warned Yemen is at a "crossroads" and urged political leaders to "take up their responsibilities and achieve consensus" as he battles for a negotiated solution.

But as the talks got underway behind closed doors in a Sanaa hotel Monday, the Nasserite pan-Arab party walked out from the meeting vowing not to return.

"We will not return to the table of negotiations," Nasserite party chief Abdullah Nooman told reporters.

He charged that the Houthis were insisting on holding talks based on the "constitutional declaration" under which they took over the government on Friday.

The militia move has been widely denounces as a "coup".

The Houthis "have threatened to take measures" against the Nasserite party and Al-Islah, a Sunni Islamist party whose supporters have battled the Shia militia, he added.

Al-Islah and the Nasserite party have both rejected the Houthi power grab and called for a return to political dialogue.

On Friday, the militia dissolved parliament and created a "presidential council" in a move it said was designed to fill a power vacuum after Western-backed President Abedrabbo Mansour Hadi and Prime Minister Khalid Bahah resigned last month.

The militia also sought to portray the move as a way of heading off the threat from Al-Qaeda, which has a strong presence in southern and eastern Yemen.

The Houthis seized Sanaa in September after sweeping into the capital unopposed from their northern stronghold. Despite a UN-brokered deal, they have refused to withdraw their fighters from the city.

Benomar, who announced the resumption of talks between the Houthis and Yemen's fractured parties on Sunday, has warned that time is running out for a "peaceful solution".

"The dialogue resumes from where it stopped on Thursday," he said.

But militia leader Abdel Malek al-Houthi insisted at the weekend that the Houthis would only take part in talks centred on their consitutional declaration, and demanded that their rivals "rectify their policies".

The takeover of the government by the Houthis, who have controlled the capital since last September, has drawn international condemnation, with UN chief Ban Ki-moon calling on Sunday for Hadi to be restored to power.

"The situation is very, very seriously deteriorating, with the Houthis taking power and making this government vacuum. There must be restoration of legitimacy of President Hadi," Ban said.

State news agency Saba, which is under Houthi control, reported that the president had told visitors his resignation is "irrevocable".

Hadi had been under virtual house arrest since the Houthis seized the presidential palace and key government buildings last month, prompting him to tender his resignation to parliament.

The Houthis have said they will set up a national council of 551 members to replace the legislature as well as a five-member presidential council that would form a transitional government to run Yemen for two years.

They have set up a security commission to run affairs until the presidential council is formed.

The fall of Hadi's government has sparked fears that impoverished Yemen -- strategically located next to oil-rich Saudi Arabia and on the key shipping route from the Suez Canal to the Gulf -- could plunge into chaos.