Shaken by ambushes that killed 13 soldiers in Gaza last week, the worst blow to Israeli forces since 2002, the army was preparing not only to flatten homes it believed were gun nests but possibly digging a moat to help block arms smugglers.

But Israel's clampdown has drawn flak from its key US ally as thousands of Palestinians could be made homeless, and from Palestinian officials contending that it contradicted Prime Minister Ariel Sharon's plan to "disengage" from occupied Gaza.

UN relief officials said more than 1,000 Rafah refugees were already in the street after the army bulldozed about 80 homes in initial demolitions last week, temporarily halted by a Supreme Court injunction before it was lifted on Sunday.

The ruling panicked hundreds of Palestinians to flee homes near the "Philadelphi" buffer strip along Gaza's southern border with Egypt. Early Monday Israeli tanks and troop carriers moved in to cut off access routes to Rafah, witnesses said.

Israeli helicopters fired missiles at an office of Palestinian President Yasser Arafat's Fatah movement -- which includes a militant group involved in last week's ambushes -- and another faction in Gaza City in the north of the territory.

Medics said the Fatah building and offices of the Democratic Front were empty and there were no casualties in the missile strikes, the latest in a series since last week's ambushes.

Militants killed six soldiers with a bomb laid in the path of their troop carrier in Gaza City on May 10. They blew up five more soldiers the same way in the Rafah corridor the next day. Another two soldiers were killed by sniper fire on Friday.

Meanwhile, Palestinian Prime Minister Ahmed Qurie appealed to the White House Monday for intervention to stop Israel's threatened mass demolition of homes in a Gaza refugee camp as panicked residents fled.

Qurie's plea at a Berlin meeting with US national security adviser Condoleezza Rice -- easing Palestinians' diplomatic isolation from Washington -- came after Israel sealed off Rafah refugee camp with armored forces to prepare for the crackdown.