The Kosovo Liberation Army missed a deadline today for certifying that a third of its weapons had been turned in at NATO-controlled stockades, as top NATO officials concluded that the ethnic Albanian paramilitary force had underreported its total arms holdings and needs to make a new accounting.

NATO played down the missed deadline and gave the KLA a 48-hour extension, but some NATO and KLA officials acknowledged that some militiamen and local commanders were resisting giving up so many weapons.

Today's deadline was the first of three that are supposed to lead to the surrender of all long-barreled and automatic weaponry, including AK-47 assault rifles; antitank weapons; grenades; land mines; mortars and artillery. Another third must be delivered to NATO-controlled stockades within a month, and the final third by Sept. 22.

British Lt. Gen. Mike Jackson, the commander of NATO-led peacekeeping forces in Kosovo, asked that Agi Ceku, the KLA's military leader, and other members of its leadership be taken by helicopter on Friday to all of the approved weapons storage sites so they can check the number of arms at each, NATO officials said. "Jackson told them to go back and redo their homework," one official said.

In a statement, NATO said "significant amounts" of arms had flooded into the storage sites in the past few days. "I am encouraged by the quantity of weapons that have been handed in," Jackson said. "What is needed now is the time to ensure that the accounting process has been completed correctly. Gen. Ceku wishes to be able to give me his personal assurance that the [KLA] . . . has complied."

The task of verifying compliance with the NATO-KLA demilitarization accord is complicated by the fact that the former rebels have never declared the size of their arms stockpile, and most of the weapons are small arms that NATO officials say would be impossible to track down. As a result, NATO has only been able to guess at the KLA's holdings and speculate that the amount declared today was too small.

"Some fighters will never turn in their Kalashnikovs," said one KLA official, referring to the Soviet-designed AK-47s. "They're war trophies."

The disarmament accord signed by NATO and the KLA last month left the door open for a portion of the KLA to be recruited into a new Kosovo security force. But NATO officials said they do not expect that such a force will be created, and that at best some of the former guerrillas may be hired to serve in a special wing of the new police force that is to be established by next spring.

NATO officials said the KLA has generally met the terms of the agreement, but there have been scattered breaches. In southern Kosovo Wednesday, NATO troops detained 15 KLA members who were carrying weapons and wearing uniforms two weeks after a deadline restricting the movement of uniformed rebels to designated assembly areas. The 15 were disarmed before being returned to an assembly area.

Also this week, NATO soldiers arrested more than a dozen former rebels after finding an illegal KLA-run detention center in southern Kosovo. NATO urged the KLA leadership to tighten controls over its members and to prevent further violations of the agreement the group signed.