U.S. Warns Serbs It May Send Arms to Bosnia Muslims : Balkans: Clinton’s special envoy says Western nations are becoming impatient with rebels’ rejection of peace plan for devastated republic.

The United States’ special envoy to the troubled Balkans warned Wednesday that Washington will call for sending weapons to Bosnia-Herzegovina’s outgunned government forces unless rebel Serbs accept a Western-mediated peace plan.

The warning issued by Reginald Bartholomew followed an impassioned call for decisive action by former British Prime Minister Margaret Thatcher, who said the West’s failure to stop the Bosnian bloodshed made it an “accomplice to massacre.”

Even Russia, the Serbs’ historic ally, stepped up pressure on Bosnian Serb leader Radovan Karadzic to agree to the plan.

Bosnia’s Croatian community and the Muslim-led Sarajevo government have accepted the proposal of mediators Cyrus R. Vance and Lord Owen to bring peace to the devastated republic by carving it into 10 ethnic provinces under weak central control.

But Karadzic and his sponsors in the Serbian capital of Belgrade have refused to sign on to the Vance-Owen plan, demanding independence from Sarajevo and the right to link land seized in Bosnia and Croatia to an expanded Serbian state.

Bosnia’s rebel Serbs have already conquered 70% of the republic on the strength of the considerable arsenal they have been provided by Belgrade.

The Bosnian government, which is internationally recognized and represented at the United Nations, has been unable to buy weapons to deter the Serbian siege because of a U.N. embargo imposed on the former Yugoslav federation before it broke up into separate republics.

Bartholomew, appointed by President Clinton in February to search for some way to resolve the Balkan crisis, said Western governments are becoming impatient with the Bosnian Serbs’ continued rejection of the settlement plan.

“The military and human horror has to stop now,” he said. “Bosnian Serbs have to do it now.”

He said the United States will “pursue the lifting of the arms embargo” if Serbian compliance is not soon forthcoming.

“If they (the Serbs) persist in their actions, the international community will make of Serbia a pariah state for as far ahead as we can see,” Bartholomew said.

In Washington, George Stephanopoulos, the White House communications director, complemented Bartholomew’s warnings by raising both the threat of a move to lift the arms embargo for the Muslims and the prospect of tightened sanctions against Serbia and Montenegro.

The tighter sanctions, he told reporters, “will ratchet up the pressure, and we hope that that will bring the Serbs to the table. . . . We will continue to pressure them in many different ways.”

The U.N. Security Council was to have considered a package of tighter sanctions Monday, but Russia, which has a veto over council actions, demanded a two-week delay, ostensibly to give the Serbs time to reconsider. The issue was rescheduled for April 26, one day after a crucial referendum that may decide Russian President Boris N. Yeltsin’s fate.

Angered by the decision by the five permanent council members to delay the vote, five nonaligned countries on the 15-member council tried without success Wednesday to force a vote.

They were unable to muster the nine votes necessary to force the issue.

Russia, whose turn it is to assume presidency of the council in May, has called for a council meeting on the foreign minister level to discuss the Balkan crisis, diplomats said Wednesday.

During a visit to Belgrade, Bartholomew met with Karadzic, Serbian President Slobodan Milosevic and Yugoslav head of state Dobrica Cosic, the masterminds of the Serbian drive for an expanded and ethnically pure state.

Bartholomew also met with Russia’s Deputy Foreign Minister Vitaly Churkin, who was on a similar mission to pressure the Serbs to stop their offensive.

Thatcher caused a diplomatic tempest in Britain with her fervent call for arming the Bosnian government.

“I am ashamed of the European Community, that this is happening in the heart of Europe, and they have not done any more to stop it,” she said in a British Broadcasting Co. interview. “The West, by not doing more, has been a little like an accomplice to massacre.”

Her accusation was brushed off by Owen as “simple,” and by British Defense Secretary Malcolm Rifkind as “emotional nonsense.”

But it probably struck a chord with many Europeans disturbed by shocking pictures and horror stories pouring out of the Bosnian war zones.

However, Russia and the British and French governments, who have troops in Bosnia serving as U.N. peacekeepers, have opposed talk of lifting the embargo for the Muslims.

They say the move will simply increase the bloodshed and put U.N. peacekeeping troops at even greater risk.

Also Wednesday:

* Monitors of NATO’s Operation Deny Flight, aimed at enforcing a “no-fly” zone over Bosnia, reported what appears to be the first violation of the exclusion zone since patrolling began Monday. A Dutch F-16 detected an aircraft over southeastern Bosnia on Tuesday evening, but it disappeared from radar screens before any action was taken.

* The Office of the U.N. High Commissioner for Refugees resumed an airlift of food and other supplies into Sarajevo after a five-day suspension prompted by security concerns.

* U.N. monitors reported scattered shelling in Srebrenica. Bosnian Serb artillery attacks on the town the day before killed eight and wounded 21, said John McMillan, a spokesman for the U.N. refugee agency in Sarajevo, Bosnia’s capital.

Even more ferocious shelling on Monday killed almost 60 people.

He also said that of over 600 refugees evacuated from Srebrenica on Tuesday, seven children and three adults died en route to Tuzla, a Bosnian government-controlled city 35 miles to the northwest.