GAZA CITY (AFP) - A convoy carrying U.S. diplomats observing the implementation of a peace plan was ripped apart by a road-side bomb in the northern Gaza Strip Wednesday, killing at least four of the occupants, Palestinian security officials said.

The blast went off just after 10:00 am (0800 GMT) as the convoy was traveling in the Beit Hanun area near the Erez border crossing which leads into southern Israel.

The blast is the first attack of its kind in the Palestinian territories against a U.S. target. Israeli forces stormed the area a short time later.

A U.S. diplomatic source said all the victims were American citizens but did not confirm that four people had been killed.

Top U.S. envoy John Wolf, the man appointed by President George W. Bush to oversee the implementation of the "roadmap" peace plan, was not in the convoy, added the U.S. diplomatic source, who asked not to be named.

Palestinian leader Yasser Arafat strongly condemned the attack and offered his condolences to Bush.

"I strongly condemn this crime which targetted Americans observers who came on a mission of peace and security," he said in a statement issued by his office.

Palestinian prime minister Ahmed Qorei and chief negotiator Saeb Erakat had similar words.

"We express our profound regret and our anger after this incident which we condemn and denounce," Qorei told reporters in the West Bank town of Ramallah where Arafat has his headquarters.

"We offer our condolences to the families of the victims," he added.

Erakat said the attack was "against the interests of the Palestinian people and the roadmap," stressing the victims were overseeing the implementation of the "roadmap" plan which sets out two-state solution to the conflict but has floundered amid fresh violence.