Serb Forces From Croatia Join Onslaught on Bihac : Bosnia: Tanks and 1,000 rebel troops pour into Muslim pocket. On diplomatic front, Serbia reportedly agrees to let observers monitor border with Bosnia.

Serbian forces from Croatia and Bosnia launched a major assault Thursday against the Muslim-led army in Bosnia’s northwestern Bihac enclave, while diplomats said Serbia would allow foreign monitors to check its cutoff of most assistance to the Bosnian Serbs.

As more than 1,000 rebel Serbian troops backed by tanks and artillery poured into Bihac from Croatia, Serbian forces fired a surface-to-air missile at two NATO combat aircraft, U.N. and North Atlantic Treaty Organization officials said.

The British Royal Navy Sea Harrier jump jets were not hit.

The United Nations said the Bihac pincer offensive was alarming because it involved rebel Serbs from neighboring Croatia for the first time.

U.N. spokesman Maj. Dacre Holloway, describing the Serbian thrust from Croatia as a major offensive, said, “We’re talking of upward of 1,000 troops, tanks and artillery--the works.”

NATO’s Southern Command in Naples, Italy, said the two Harriers were on a routine training mission and returned safely to their aircraft carrier, the Invincible, in the Adriatic Sea.

“They were operating in a close air-support training mission . . . when they visually sighted a small surface-to-air missile coming toward them. Both planes took proper evasive action, and they returned to their ship safely,” NATO said.

In Paris, the French Foreign Ministry said Thursday that Serbia has agreed to let foreign observers monitor the closing of its border with Serb-controlled parts of Bosnia-Herzegovina.

Serbia agreed to allow “an international humanitarian mission” along the border, ministry spokesman Richard Duque said. The mission would monitor implementation of the Belgrade government’s pledge to cut off all assistance, except food and medicine, to Bosnian Serbs waging war against the Muslim-led Bosnian government.

Duque said Serbia disclosed its agreement Wednesday at a meeting in Berlin of officials from the United States, Russia, Germany, Britain and France. The five countries’ latest plan for ending Bosnia’s 2 1/2-year war was rejected by the Bosnian Serbs last month.

Duque gave no details about the composition of the observer team. Serbian President Slobodan Milosevic has been under pressure from ultranationalists not to accept foreign military observers, and it appeared from Duque’s phrasing that a compromise had been reached.

There was no immediate official reaction in Belgrade, the Serbian and Yugoslav capital, to the statement.

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The United States and the major European powers have offered to lift some economic sanctions on Yugoslavia, now reduced to the republics of Serbia and Montenegro, if the Serbian-led government allows international inspectors to verify that it has cut off aid to Bosnian Serb forces.

As an initial result, civilian air traffic to Belgrade could be reopened within a few weeks and Yugoslavia readmitted to international sports and cultural events, with other measures to follow.

The objective of such a move is to increase pressure on the Bosnian Serbs by separating them from Serbia, their main ally.

But some Bosnians fear that the plan will enable the Bosnian Serbs to continue receiving clandestine aid while allowing Serbia itself to recover.