Nato began detailed contingency planning yesterday for the withdrawal of United Nations peace-keeping forces from Bosnia, a decision that may herald the end of the UN Security Council arms embargo against the Muslim-led Bosnian government.

Nato ambassadors in Brussels approved a request from military authorities to prepare for the removal of UN forces from Bosnia in hostile conditions.

The measure does not mean that Britain, France and other countries with soldiers in Bosnia have taken a decision to pull out the 23,000 UN troops. But Douglas Hurd, the Foreign Secretary, told the Commons: ''As with all military operations, planning is in hand to cover a variety of eventualities, including withdrawal.''

Nato officials said at least 20,000 troops would be needed to evacuate the UN forces, and the operation could take several months. It was important to establish now which countries were ready to provide troops for such an operation, and to plan their specific duties, they said.

The Clinton administration, locked in disputes with Britain, France and Russia over strategy in Bosnia, has made clear that the United States will take a leading role in evacuating UN troops if necessary.

Britain and France say that UN peace-keepers will have to leave if the arms embargo is lifted because they could no longer perform humanitarian tasks and would become potential targets for Bosnian Serb forces.

France's Foreign Minister, Alain Juppe, confirmed that his government had asked Nato and the UN to plan in detail for a UN withdrawal. In an implicit rebuke to the US, he said: ''I say today that the obstinacy of some and the demagogy of others risks setting the Balkans ablaze tomorrow. I am still ready to do everything I can to prevent that. But it is my duty to say that it is, alas, no longer unlikely.''

It is still not certain that the arms embargo will be lifted and, if so, when. The new Republican-dominated US Congress does not meet until next month, and it is already clear that the Republican Party is far from united on the issue. In addition, the Clinton administration has to take account of the fact that ending the embargo would cause even more serious rifts inside the Atlantic alliance and with Russia than have appeared in the last two weeks.

UN officials said that 400 out of 1,200 Bangladeshi troops were being withdrawn from the Bihac ''safe area'' of northern Bosnia because of resupply problems caused by the Serbian grip on the enclave.

The Bosnian Serb leader, Radovan Karadzic, said UN troops could leave Bosnia without hindrance ''if they take their weapons with them and don't leave them in the hands of the Muslims''. He added a Western-Russian offer to the Bosnian Serbs of confederation with Serbia was unacceptable. He insisted on a Bosnian Serb state.

Further reports, page 12

Leading article, page 17