NAIROBI, KENYA, JAN. 3 -- Somali rebels today rejected President Mohamed Siad Barre's call for a cease-fire, dashing Italy's hopes of a quick evacuation of Westerners trapped in the embattled capital of Mogadishu, where street fighting was reported heavy for a fifth day.

The rebels of the United Somali Congress said they had massed reinforcements in the capital and were preparing a "final offensive." The "capture of Siad Barre is only a matter of time," a communique said.

Siad Barre, accused by the rebels of running a "cruel, corrupt" government for the last 21 years, was driven from the presidential palace by rebels on Monday and is believed holed up in a fortified bunker at the seaside airport south of Mogadishu.

{A Reuter news agency photographer who flew into Mogadishu Wednesday night said clouds of smoke hung over the airport and heavy artillery battles were taking place nearby.}

The rebels rejected a plea for a truce issued Wednesday by Siad Barre and set terms for an evacuation of foreigners, estimated to number more than 500, most of them Italians.

The United Somali Congress said it accepted the need for evacuation "but only under the supervision of the international Red Cross or similar bodies." It said plans by Italy and other Western nations to evacuate their citizens "could be interpreted as an intervention" on behalf of Siad Barre's government. The rebels also rejected any negotiations or cease-fire with the president, apparently even a pause in the fighting that would permit evacuees to assemble and be led safely to the airport or harbor.

Two Italian military transport planes, one of them carrying a small contingent of paratroops, arrived in Nairobi today to assist in the evacuation of Italians and other Westerners. Italian officials in Rome said the troops had been sent only to provide security during an evacuation.

A Canadian diplomat in Nairobi said about 40 Westerners had fled by boat from Brava, south of Mogadishu, and were headed for Kenya.

Staff writer John M. Goshko added from Washington:

State Department spokesman Richard Boucher said Voice of America broadcasts were urging the estimated 85 Americans in Somalia to stay in touch with the U.S. Embassy and, if they are able to move safely, to seek refuge there.

"At this point, we're still working on evacuation plans and haven't been able to put anything into effect, given the security situation," Boucher said.

An Italian government source in Washington said the Red Cross had agreed to lend its name and facilities to an evacuation of Westerners from Mogadishu. An Italian navy frigate, "one of the ones in the {Persian} gulf," was en route to Indian Ocean waters off Mogadishu to assist in any evacuation, he said.