George Bush and Tony Blair made the announcement after meeting to discuss the crisis in the Middle East in Washington on Friday.
"We agree that a multinational force must be dispatched to Lebanon quickly to augment the Lebanese army as it moves to the south of that country. An effective multinational force will help speed delivery of humanitarian relief," Bush said.
Blair said world powers would meet at the UN on Monday to discuss the possible deployment of a UN "stabilisation force" for Lebanon.
Bush said Condoleezza Rice, the US secretary of state, would return to the Middle East on Saturday to hammer out with Israel and Lebanon a new UN resolution supporting the force.
"Her instructions are to work with Israel and Lebanon to come up with an acceptable UN Security Council resolution that we can table next week," Bush said.
Blair said it was important not only to get a cessation of violence but to use the opportunity to set out and achieve a "different strategic direction for the whole of that region".
The British prime minister also warned both Syria and Iran that they should participate as responsible members of the international community "or face the risk of increasing confrontation".
The Lebanese authorities say up to 600 civilians have been killed during the 17-day Israeli onslaught.