The Yemeni capital's airport was shut down on Saturday after forces loyal to a sacked general close to former president Ali Abdullah Saleh encircled it and threatened to shoot down planes, an airport official said.

"No aircraft has taken off or landed since these forces made their threat late on Friday," said the official, adding that the troops surrounding the airport were backed by members of the Hamdan tribe that supports former strongman Saleh.

Another official told AFP nine international and seven domestic departing flights had been cancelled, while three incoming Yemenia Airways flights were diverted to the main southern city Aden.

The airport has been encircled by forces loyal to air force chief General Mohammed Saleh al-Ahmar, Saleh's half-brother, who has refused to step down after being sacked by President Abdrabuh Mansur Hadi, the source said.

The men were led by Naji Jamaan, a Hamdan tribal chief, the source added.

On Thursday, the UN Security Council expressed concern over recent events in Yemen, where followers of Saleh have been accused of hampering the political transition.

Ahmar, in a message to his troops, has refused to go unless the defence minister and other senior officials also step down, a military source said.

He also demanded that several members of the powerful Hashed tribe, which backed defectors during last year's anti-regime protests, be forced into exile.

In addition to Ahmar, Hadi on Friday also sacked Saleh's nephew, General Tareq Mohammed Abdullah Saleh, who heads the presidential guard.

The military source said Tareq Saleh had likewise refused to quit and turned down an offer to take command of the 37th Battalion of the Republican Guard in the southeastern province of Hadramawt.

At the same time, Hadi sacked General Mohammed Ali Mohsen al-Ahmar, leader of units in the eastern region who is loyal to General Ali Mohsen al-Ahmar, together with whom he defected to the anti-Saleh opposition last year.

A military official told AFP later on Saturday that Saleh's half-brother was "still present at the air force base in Sanaa protected by Republican Guard troops" commanded by Saleh's son Ahmed.

Gulf Cooperation Council chief Abdullatif al-Zayani, who led mediation efforts to convince Saleh to step down, said the six-nation group "supports" Hadi and "backs all measures he takes to help Yemen exit its current crisis."

Zayani "urged all political power players in Yemen and all those involved to support the Yemeni president to move forward in implementing the principles stipulated by the Gulf initiative."

That initiative, under which Saleh resigned following nationwide protests after 33 years in power in exchange for immunity from prosecution, includes a restructuring of the army as one of several conditions.

Saleh's son Ahmed still heads the elite Republican Guard, while a nephew, Yehya, commands central security services.

In a February speech, President Hadi stressed the need to reunify the army as he pledged "radical reforms" and to fight Al-Qaeda as he outlined a two-year transition plan.

Last month, he named General Salem Ali Qatan to head the 31st Armoured Brigade in southern Yemen. He replaces General Mahdi Moqala, known for his close ties to Saleh and accused of corruption.

Two days later, Al-Qaeda militants attacked troops in the southern city of Zinjibar and killed 185 soldiers of them. Survivors accused some army leaders who had served under Saleh of "collaborating" in the attack.

In a statement on Thursday, the Security Council expressed "concern at the recent deterioration in cooperation among political actors and the risks this poses to the transition."

It called on all sides in Yemen "to remain committed to the political transition, constitutional order, to play a constructive role in the process and to reject violence."

The council also expressed "strong concern about intensified terrorist attacks, including by Al-Qaeda, within Yemen."

Saleh retains the leadership of the General People's Congress, and aides have not ruled out his standing in a contested presidential election due to be held alongside new parliamentary polls in 2014.