WASHINGTON — President Bush and British Prime Minister Tony Blair called Friday for an urgent U.N. resolution backed by international peacekeepers to help end the fighting between Israelis and Hezbollah militants in southern Lebanon.

Both stopped short of endorsing an immediate cease-fire, however, insisting that Hezbollah militias must be prevented from controlling southern Lebanon if the most intensive Arab-Israeli warfare since 1982 is to end.

"Now's the time to be firm," said Bush. "Hezbollah and its Iranian and Syrian sponsors are willing to kill and to use violence to stop the spread of peace and democracy. And they're not going to succeed."

Blair, standing beside Bush in a White House news conference after an Oval Office meeting on the conflict, echoed the essence of the president's position, even as he stressed the need to end fighting that has killed nearly 500 people in the past 16 days.

"We want it to stop, and we want it to stop now," said Blair. "But the brutal reality of the situation is that we're only going to get violence stopped and stability introduced on the basis of clear principles."

Hoping to nail down the key markers, Bush directed Secretary of State Condoleezza Rice to return to the region, just days after she left, to hammer out the terms of a U.N. resolution both governments could accept. Rice was to arrive in Jerusalem late today. While in the region, she is to meet with Israeli Prime Minister Ehud Olmert and his counterpart in Lebanon, Fouad Siniora.

Bush said that quickly setting a clear framework for ending hostilities and mandating a multinational force "will make possible what so many around the world want to see: the end of Hezbollah's attacks on Israel, the return of Israeli soldiers taken hostage by the terrorists, the suspension of Israel's operations in Lebanon, and the withdrawal of Israeli forces."

At the U.N. headquarters in New York, Jan Egeland, the under-secretary-general for humanitarian affairs, called for a three-day cease-fire to allow aid workers to deliver food and medical supplies to thousands of civilians in Lebanon who have been stranded by the fighting.

On the ground, though, the conflict raged. Hezbollah militia units claimed to have launched a rocket — the Khaibar-1 — more powerful than any of the more than 1,500 the militants have launched into Israel since the conflict began. Israeli officials said the rocket was likely an Iranian weapon with a range of 45 miles, capable of threatening the Tel Aviv suburbs and other areas far deeper into Israel than the shorter-range Katyusha rockets Hezbollah has previously relied on.