SARAJEVO -- Thousands of Bosnian army soldiers massed north of Sarajevo, triggering fears Wednesday of a major escalation in the 38-month Bosnian war.


The United Nations, which has reported up to 30,000 Bosnian army soldiers had moved into the area around Breza north of Sarajevo in the biggest deployment of the war, said it would try to head off any offensive by the army.


News of the troop buildup came as NATO was reported to be close to giving formal approval to a contingency plan to pull UN peacekeepers out of Bosnia if the situation deteriorates.


British Foreign Office minister Douglas Hogg said an attack by Bosnian government forces against the Serbs could lead to a withdrawal of peacekeepers.


In Sarajevo UN spokesman Lieutenant Colonel Gary Coward, asked if the UN special envoy Yasushi Akashi would try to dissuade the Bosnians from mounting an offensive, said, "Yes, I think in essence because our instructions are to stabilize the situation."


In Vienna, Bosnian Foreign Minister Muhamed Sacirbey said Serbs had moved new weapons into the UN exclusion zone around Sarajevo and the government army had to take "preventive measures" now.


He refused to confirm or deny reports that up to 30,000 Bosnian government troops were gathering north of Sarajevo, possibly to try to break the Serb siege of Sarajevo.


"We must be prepared not to liberate Sarajevo but actually to defend Sarajevo at this time," he said.


The UN may be trying to head off a move by the Moslem-led army to attempt what the international community has been unable to do -- to end the Serb siege and the shelling of civilians.


A Bosnian Serb army commander warned Wednesday his soldiers would fiercely oppose any attempt to break the siege, the Bosnian Serb news agency SRNA reported.


While suggesting reports of a big buildup by government troops were exaggerated, Major General Dragomir Milosevic, commander of the Serb army corps besieging the Bosnian capital, made clear any attack would be met with a strong counteroffensive. At Bosnian Serb headquarters in the mountain village of Pale, outside Sarajevo, cafes were full of talk that the Moslem-led army was massing 20 kilometers north of the Bosnian capital.


Bosnian Serb sources said privately they were more concerned with the improving military strength of the government army than the threat of any Western military action mounted by NATO or a UN peacekeeping mission.


"They [the Bosnian army] have been arming themselves steadily," said one Serb source. "They are much stronger then they were before." News of the troop buildup overshadowed the release of most of the UN peacekeepers held hostage by the Bosnian Serbs in retaliation for NATO air strikes last month.


Twenty-six mainly British and French soldiers left Novi Sad on Wednesday and were due to fly to Croatia. A UN spokesman said 26 UN peacekeepers were still unaccounted for.