ZENICA, BOSNIA, MARCH 3 -- Serb forces besieging the north-central enclave of Maglaj rejected a U.N. request today to bring a convoy of food to the last Muslim pocket in Bosnia cut off from humanitarian aid.

The Serb rebuff came despite an announcement last week by the U.N. command in Bosnia that it would no longer wait for any of the former Yugoslav republic's three warring sides to give permission for humanitarian convoys to cross battle lines. It occurred against the backdrop of a fundamentally changed strategic situation in central Bosnia, where Muslim and Croat forces have stopped fighting each other and can turn their attention to their Serb enemies.

U.N. officials said that for the time being they would not contest the Serb rejection but announced that an armored platoon of British Light Dragoons was being sent to the area to help smooth the convoy's crossing "at a later date."

However, a top U.N. aid official warned that any delay in food aid to the region could be disastrous. Larry Hollingworth, chief of operations in central Bosnia for the U.N. High Commissioner for Refugees, described Maglaj as "perhaps the worst place in Bosnia."

"The Serbs should not be allowed to decide when we can feed Maglaj," he said. "If there's ever a place where the U.N. needs to show its new toughness, then it's right there. These people are skin and bones, and they need food."

U.N. officials who visited the enclave in mid-January estimated that at least 30 percent of the population was malnourished. Since July, Maglaj and its sister city, Tesanj, have survived on food airdropped by the U.S.-led program known as Operation Provide Promise.

Croat and Serb fighters have only allowed two U.N. food convoys into Maglaj and Tesanj since July, when a combined Croat-Serb offensive severed the pocket from Muslim-held parts of central Bosnia.

Since Monday, however, Croat forces have stopped fighting Muslims in central Bosnia and, around Maglaj, have told the United Nations that the convoy can cross their lines. Serb units, however, which were not party to Monday's cease-fire, continue to refuse to allow U.N. food trucks access to the enclave, home to about 19,000 people.

On Sunday morning, Hollingworth said, a brigade of Croat militiamen who had been fighting alongside Serb forces around the pocket pulled out, thwarting the Serbs' latest offensive.

The Serbs "want to take Maglaj. Now that they can't do it by force, they want to starve it out," said Hollingworth. "We've allowed them to do this to other places in Bosnia. We can't allow them to do it again."

Under the U.N. mandate in Bosnia, peacekeeping forces have authority to push humanitarian convoys into isolated areas by force, but have never used it. The new commander of U.N. forces in Bosnia, British army Lt. Gen. Michael Rose, has hinted that under his more robust interpretation of the U.N. mission here, he would consider employing force.