U.N. officials confirmed Saturday that more Bosnian Serb artillery had been spotted around Sarajevo, and said the Serbs might be testing enforcement of the cease-fire.
"It is obvious that there still are some heavy weapons not under our control," said Maj. Rob Annink, a spokesman for United Nations peacekeepers.
He was referring to disclosures that peacekeepers found six Serb 122mm howitzers late Thursday just inside the NATO-mandated 12-mile exclusion zone around the Bosnian capital.
Annink said five of the howitzers had been pulled out of the zone in compliance with U.N. demands, and the last was to be removed Saturday.
But the discovery of the artillery, along with more gunfire and grenade exchanges between Bosnian Serbs and the Muslim-led government troops defending Sarajevo, raised fears that the three-week cease-fire could be in trouble.
U.N. officials did not make clear whether Serbs recently had moved the six howitzers back into the zone, or whether the guns simply were discovered belatedly.
NATO has threatened to bomb any heavy weapons not pulled back from Sarajevo or put under U.N. control. The air strikes would have to be requested by the United Nations.
Despite numerous apparent violations since the ultimatum on artillery passed two weeks ago, U.N. officials say the Bosnian Serbs have mostly complied, making air strikes unnecessary.
But with major powers reluctant to supply thousands more troops to police the fragile cease-fire, concern mounted that Serbs might be testing the international community's resolve.
"It looks like that," Annink said. "And that's why we need, very fast, extra troops to decrease this tension that is building up."
Elsewhere in Bosnia, peacekeepers reported four people killed and three wounded by four shells that hit the town of Zenica, 34 miles northwest of Sarajevo, where there is a main U.N. aid distribution depot.
A U.N. official at the British base in nearby Vitez said the shells were thought to have been fired from Bosnian Serb positions.
The U.N. Security Council unanimously passed a resolution Friday calling for restoration of public services in Sarajevo and a final lifting of the Bosnian Serb siege, including free movement of people and aid.
But more problems with U.N. aid convoys were reported, including one bound for Sarajevo that has been blocked for days by protesting Serb women in Hadzici, west of the Bosnian capital.
Serbs also continued to deny clearance for an aid convoy to the northern Muslim enclave of Maglaj, reported to be under heavy bombardment. "The bottom line is that they are just not letting this convoy in, and using all kind of excuses," said U.N. relief spokesman Kris Janowski.
In central and southwestern Bosnia, a weeklong cease-fire held between Bosnian Croat and government troops.