A NATO warplane buzzed a U.N. weapons depot on Sarajevo's northern outskirts and chased away Bosnian Serbs trying to retrieve heavy weapons, U.N. officials said Monday.

U.N. spokesman Cmdr. Eric Chaperon said 10 to 15 Serbs approached the arms collection point guarded by French peacekeepers Sunday night, demanding to take a 122mm artillery piece.A four-hour standoff ended peacefully around midnight when the plane requested by peacekeepers buzzed the area around the depot in Poljine, just north of the capital's main Kosevo hospital, and the Serbs left.

"It was a test," said Maj. Guy Vinet, spokesman for U.N. troops who patrol Sarajevo. "After the incident, we thought that it was not very serious, but during the incident, yes, it was serious."

U.N. troops have had heavy weapons under guard at various points around Sarajevo since a NATO ultimatum in February for the Serbs to remove weapons from a 12.4-mile radius around Sarajevo or turn them over to U.N. troops.

There were about 30 pieces of weaponry at the collection point, guarded by about two dozen French soldiers, Vinet said.

Spokesmen also reported more fighting between Serbs and troops of Bosnia's Muslim-led government near the boundary of the two-mile radius around the eastern enclave of Gorazde, from which Serb soldiers are supposed to have withdrawn.

Few details were immediately available.

U.N. officials said Sunday too many Serb militiamen remained inside Gorazde's exclusion zone a week after NATO's ultimatum to remove them.

Chaperon on Sunday reported increasing tension in Gorazde.

Members of the Serb militia or police are not specifically mentioned in the NATO ultimatum. But U.N. officials expressed concern at their presence, and there have been reports that Serb soldiers returned to the zone with police uniforms and weapons.

Chaperon estimated Monday that about 100 Serb policemen remained in the area.

The Gorazde enclave, under siege by Serbs for most of the 2-year-old conflict, is home to 65,000 residents and refugees, most of them Muslims. The U.N.-designated "safe area" is 35 miles southeast of Sarajevo.

Skirmishes were reported Sunday on many fronts in Bosnia.

Bosnian radio said seven people were wounded in Serb shelling of the Tesanj region northeast of Sarajevo, and that Serbs shelled Zavidovici, about 12.5 miles to the south.

The Yugoslav news agency Tanjug and the Bosnian Serb agency Srna said seven people were killed by government shelling of Doboj in northern Bosnia and villages near Vlasenica, 40 miles northeast of Sarajevo.

None of the local news reports could be independently confirmed.

Bosnian Serb military officials also renewed their charge that U.N. peacekeepers were taking the government's side. Chief of Staff Gen. Manojlo Milovanovic alleged that in an exchange of fire, the U.N. force made "its first land strike on Serb positions and civilian settlements in the former Bosnia-Herzegovina" and killed nine people.

Chaperon said the Serbs had fired four anti-tank missiles at Danish Leopard tanks near the northern city of Tuzla, as well as artillery and mortar rounds. The tanks returned fire with 72 tank rounds. There were no U.N. casualties and no damage reported.

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Additional Information

2 Americans killed

Two American photographers were killed and an American writer was injured when their car ran over a landmine near Mostar, Bosnia-Herzegovina.

Bryan Brinton, accredited to Magnolia News, a Seattle weekly, died in Sunday's blast, according to U.N. officials. Also killed was Francis William Tomasic of Bloomington, Ind., who was working for Spin magazine.

William T. Vollmann, a novelist who is a senior contributing writer for Spin, was slightly injured by the blast, said Maj. Antonio Albariz, a spokesman for Spanish U.N. troops in nearby Medjugorje.

Spin spokesman Jeff Raban in New York said Tomasic was Vollmann's photographer and interpreter. He said Vollman, 34, of Sacramento, Calif., was in stable condition.