LONDON, May 30 (Reuter) - Prime Minister John Major said on Tuesday that withdrawal of British troops from Bosnia was not his government's objective.

But he added that there may be no alternative if it became impossible for U.N. peacekeepers to continue their mission.

Britain is sending 1,200 extra soldiers to Bosnia and has a 5,000-strong air mobile brigade on stand-by, sparking speculation that they were being sent to defend against attack during any withdrawal.

An advance party left Britain for Split in Croatia on Tuesday morning.

But Major told a news conference in London that a pullout of British forces serving with the United Nations Protection Force (UNPROFOR) is "not, not our objective".

But he said that if it became impossible for U.N. troops to continue their mission, Britain and its allies "may find ourselves in the position where there is no viable alternative to bringing the protection forces home".

Major said that Britain would do "everything in our power" to ensure that 33 British soldiers held hostage by Bosnian Serbs were released unharmed.

The prime minister said he had sent a message to Bosnian Serb leader Radovan Karadzic and Serb army commander General Ratko Mladic holding them "personally responsible for the safety of British troops in Bosnian Serb hands".

He warned that taking hostages was the path to "total international isolation and pariah status".