More than half the hostages on board a hijacked Afghan airliner at Stansted Airport have been released after four days of negotations.

Police say talks with the hijackers have reached a critical stage.

The Afghan airliner was hijacked on Sunday during an internal flight in Afghanistan and flown across Russia and Kazakhstan.

About 85 people - including all 21 of the children on board - left the Ariana Boeing 727, which has been on the tarmac at Stansted since Monday morning, at about 0315GMT on Thursday.

A number of those departing the plane had their hands raised.

But the steps to the airliner were then retracted with about 70 people apparently still on board the plane.

At a news conference, Essex Police's Assistant Chief Constable Joe Edwards said: "After three days of constant negotiation I am delighted to say that about 85 hostages have been released by the hostage takers."

"Women and children left the aircraft first. All those released will now be taken to a safe location and undergo medical checks. We also have to verify that all those who left are indeed hostages."

Mr Edwards said: "Talks are continuing with people on board in an attempt to secure the release of the remaining passengers and crew."

He revealed that police negotiators had held face-to-face talks with two of the kidnappers.

The drama began at about 0215GMT when a light came on in the plane.

A man then came down onto the tarmac and picked up a box, a development which police described as "significant".

Two people in traditional dress left the plane a short while later.

Earlier police said there was "definite cause for optimism" after a day of talks with the hijackers.

Mr Edwards had said he felt "much more hopeful" of a peaceful conclusion than he had on Tuesday night, when four members of the crew enraged the hijackers by escaping the plane.

Asylum speculation

There has been intense speculation that the hijack, longest in UK history, was a mass bid for asylum in the UK.

But Mr Edwards said it was more complex and added: "The detail of the negotiations are about what is the key to getting everybody off that plane willingly and unharmed and that is a complex situation."

The head of Ariana, Mullah Hamidullah, said a steward who had been released during a stopover in Tashkent, Uzbekistan, said there appeared to be a large group on the aircraft of up to 40 men, women and children who could be involved in an attempt to get political asylum.

Click here to watch live coverage of the hijack.

An official from the United Nations High Commission for Refugees has arrived at the airport, although spokesman Paul Wilkinson refused to comment on whether political asylum was the reason for the hijack.

Under international law the UK is obliged to consider any asylum claim submitted once any potential criminal proceedings are completed.

In Afghanistan, the ruling Taleban movement has arrested 10 people in charge of security at Kabul airport over the hijacking.

Also:

Pilots criticised for fleeing
Hostages 'face horrific ordeal'
Who are the hijackers?
Negotiating with hijackers
Stansted's hijack history