STANSTED, England (AP) _ Hijackers holding at least 150 people aboard an Afghan airliner freed another hostage today after he complained he was unwell, while British authorities warned the standoff could last ``for days.''

Since landing Sunday at Stansted airport, 25 miles north of London, the hijackers have requested only food, water, unspecified medical supplies and that the lavatories on the Ariana Airlines plane be emptied, police said.

More such supplies were delivered today, including omelets for breakfast.

``Negotiations remain our favorite option,'' Joe Edwards, an assistant chief constable of the Essex Police, said Monday. ``I will say it could be a very protracted technique. It could go on for days.''

The hijackers, have released nine hostages since landing at Stansted.

Exactly what the hijackers were negotiating for was unclear. A diplomat familiar with earlier negotiations when the airliner landed in Moscow said those holding the plane sought to free a prisoner held in the southern Afghan city of Kandahar.

Afghan media speculated the hijackers were dissidents who want the release of Ismail Khan, a former regional governor who has been held since 1997 by Afghanistan's ruling Taliban movement. Khan is a member of Afghanistan's opposition alliance, which still rules roughly 10 percent of the war-shattered country.

Afghan opposition leader and former President Burhanuddin Rabbani today denounced that speculation.

``The legal government of Afghanistan categorically denies its involvement in the plane hijacking,'' Rabbani said in Dushanbe, Tajikistan. ``Rumors that terrorists demand the release of the former governor of the Herat province Ismail Khan are groundless, and it's the Taliban itself that is spreading those rumors.''

Edwards said police were in contact with the plane throughout the night and that both the hostages and the hijackers remained calm.

``The people we are talking to are remarkably calm, and that's really important,'' he said. ``The atmosphere is one of control, people behaving rationally. That's going to help an awful lot.''

The Boeing 727 had been headed to the northern Afghan city of Mazar-e-Sharif, but after hijackers seized control it began a meandering journey across the former Soviet Union, stopping in Uzbekistan, Kazakstan and Russia before ending up in Britain.

The hijackers have released 31 passengers and are believed to still be holding 156 hostages of unknown nationalities. According to some of those released, the hijackers include eight young men, armed with pistols, grenades and daggers.

``They were all young _ around 25-30 years old, dressed in traditional Afghan costume _ and to begin with they were threatening,'' Mohammad Bashir Mahal, one of 10 passengers set free in Moscow, said at a news conference Monday.

``But slowly, their aggression subsided and they treated us quite well. No one was beaten or insulted. Relations were good.''

The hijackers appeared to be Afghan because of their language and clothes, he said.

Another released hostage said that 20 minutes after taking off from the Afghan capital, Kabul, early Sunday, the hijackers calmly asked passengers to put their hands on their heads.

John Broughton, another assistant chief constable of the Essex Police, said that eight captives released Monday at Stansted Airport told officials ``they were very well-treated while they were on board.''

Hamidullah, a top official for Ariana airlines, said in Kabul that all the passengers were believed to be Afghans.

The Taliban have ruled out negotiating with the hijackers.

``We do not have a message to the hijackers,'' said Gen. Rahmatullah Safi, the Taliban's representative in Europe who was at Stansted. ``We are not willing to give way to terrorism.''