Palestinians in Gaza believed Israel had called a 48-hour lull in retaliatory attacks with Hamas when Israel Air Force warplanes launched a massive bombardment of militant installations in the Gaza Strip, a UN official said Monday.

Karen Abu Zayd, commissioner of the UN Relief and Works Agency which helps Palestinian refugees, raised the possible violation of an informal truce in a video press conference with UN reporters from her base in Gaza.

Israel's UN Mission referred any comment on the reported lull to Prime Minister Ehud Olmert's office in Jerusalem. Olmert's office did not answer telephone calls for comment early Tuesday morning.

Abu Zayd said Palestinians in Gaza were surprised when Israeli warplanes sent more than 100 tons of bombs crashing down on key security installations in Hamas-ruled Gaza starting Saturday morning because it was in the middle of the lull.

The offensive began eight days after a six-month truce between Israel and the militants expired. During that time, the Israel Defense Forces said Palestinian militants fired some 300 rockets and mortars at Israeli targets, and 10 times that number over the past year.

Israel had sent mixed signals on Friday regarding its plans for Gaza. Israeli defense officials said politicians had approved a large-scale incursion into the territory. But at the same time, Israel appeared open to international pressure against an invasion, prying open its border with Gaza to allow deliveries of humanitarian aid.

"What we understood here (was) that there was a 48-hour lull to be called, and this was called by the Israelis," Abu Zayd said. "They said they would wait 48 hours. That was on Friday morning, I believe, until Sunday morning, and that they were going to evaluate."

"There was only one rocket that went out on Friday, so it was obvious that Hamas was trying, again, to observe that truce to get this back under control," she said.

"Then, everything got loose on Saturday morning at 11:30 a.m. We were all at work and very much surprised by this," Abu Zayd said.

When the Israeli offensive began, neither Defense Minister Ehud Barak nor Prime Minister Ehud Olmert made any mention of a lull.

Abu Zayd mentioned the lull when she was asked whether the population of Gaza was aware that this was all commenced by the Hamas government unilaterally ending the cease-fire and firing rockets.

"I don't think they think the truce was violated first by Hamas," she said.

"I think they saw that Hamas had observed the truce quite strictly for almost six months, certainly for four of the six months, and that they got nothing in turn - because there was to be kind of a deal," Abu Zayd said.

"If there were no rockets, the crossings would be opened," she said. "The crossings were not opened at all."