UN envoy to Yemen Jamal Benomar meanwhile arrived in the violence-wracked country and was expected to head a meeting of political rivals later in the day, a source close to the Moroccan diplomat said.

President Abdrabuh Mansur Hadi`s abducted chief of staff remained in the hands of the Huthi militia, which took control of most of Sanaa in September after sweeping south from its stronghold in the northern highlands.

Under a nine-point deal struck late on Wednesday, the militia pledged to withdraw from government buildings they seized this week during two days of violence that left at least 35 people dead and dozens wounded.

In return for concessions over a disputed draft constitution, they agreed to vacate the presidential palace, free top presidential aide Ahmed Awad bin Mubarak, withdraw from areas surrounding the residences of Hadi and Prime Minister Khalid Bahah, and abandon checkpoints across the capital.

The Huthis agreed with Hadi to "normalise" the situation in Sanaa, calling for people to return to work and schools to reopen.

However, on Thursday the terms of the agreement had yet to be implemented.

"The Huthis were expected to release Mubarak by now but he has not been freed yet," a presidency official said.

Heavily armed Huthis were out in force across the capital and around the presidential palace, an AFP correspondent reported.

Militiamen were also deployed around Hadi`s residence and even extended their presence towards the intelligence headquarters, a kilometre (about half a mile) away.

They continued to block one of the three entrances to the residence of Bahah, whose whereabouts remained unknown Thursday a day after the Huthis allowed him to leave following a two-day siege.

Gulf foreign ministers had on Wednesday accused the Huthis of an attempted "coup" and expressed support for Hadi.The situation escalated on Saturday when the militiamen seized Mubarak in an apparent bid to extract changes to a draft constitution which the Huthis oppose because it would divide Yemen into six federal regions.

The militiamen say it would split the country into rich and poor areas.

Yemen, a key US ally in the fight against Al-Qaeda, has been riven by instability since an uprising forced strongman Ali Abdullah Saleh from power in 2012.

Saleh has been accused of backing the Huthis -- who are from the same Zaidi sect of Shiite Islam as the ex-leader -- as has Shiite-dominated Iran.

The turmoil has raised fears that Yemen, which neighbours oil giant Saudi Arabia and lies on the key shipping route from the Suez Canal to the Gulf, may become a failed state similar to Somalia.

Washington has been closely monitoring the crisis and officials said Huthi gunmen had opened fire on a US diplomatic vehicle at a checkpoint near the American embassy late Tuesday.

The air and sea ports in Yemen`s second city Aden were reopened Thursday, a day after they were closed due to the unrest in the capital.

Shops in Sanaa also resumed business and people were back on the streets. But tensions were running high and Sanaa University remained shut.

"Sanaa is over," said Mohammed al-Usaimi, a 45-year-old construction worker who lives near Hadi`s residence.

"There`s no more security and no more work regardless of what they say about the return of life back to normal. This will not happen." 

Hundreds protested outside Sanaa University calling for a "new revolt" and chanting: "No to coups!"

Since overrunning the capital the Huthis have pushed farther south and east, meeting fierce resistance by Al-Qaeda and Sunni tribal fighters.

Huthi militiamen on Thursday killed two tribesmen and wounded six others in Marib, an oil-rich province in eastern Yemen on which the movement has set its eyes. 

The Huthis have long complained of marginalisation and fought six wars with the government between 2004 and February 2010.

Wednesday`s deal gives them a greater presence in state institutions and more government posts.

AFP