U.S. officials adamantly denied any coordination with Israel -- which attacked 20 minutes after the monitors left -- and urged all sides to maintain calm. "Such accusations are baseless and ignore the facts, quite frankly," State Department spokesman J. Adam Ereli said.

U.S. officials said yesterday that the Israeli seizure of six Palestinian inmates came after months of fruitless discussions with Palestinian officials to bolster security to protect U.S. and British monitors at the Jericho prison, leaving them no choice but to abandon the facility even though they realized the action might undermine the Palestinian Authority.

U.S. officials added that they purposely did not make public their concerns because they believed publicity would have made the U.S. and British personnel the targets of attacks. "That is the price we pay," said a senior administration official, speaking on the condition of anonymity. "Our first responsibility is the safety of American and British monitors."

"Clearly, there is some sort of coordination," Arab League Secretary General Amr Moussa told al-Jazeera, which provided extensive televised coverage of the assault and of Israeli troops ordering Palestinians to strip to their underwear.

Acting on an anonymous tip, police found a 6-by-8-meter (yard) hole in a empty field. It contained at least 29 dead men most of them in their underwear in Kamaliyah, a mostly Shiite east Baghdad suburb.

Edward G. Abington Jr., a consultant to the Palestinian Authority, said Israel's action will feed the perception that Palestinian President Mahmoud Abbas is increasingly powerless and irrelevant. "This is a seismic event that will have real repercussions," Abington said. "This just totally undercuts Abu Mazen," as Abbas is known.

The senior administration official did not disagree with Abington's assessment but said they "were left with no choice." He said U.S. officials had tried to not embarrass the Palestinian Authority or Abbas over their concerns, but the Palestinians had failed to act.

The "situation really, truly was becoming untenable," he said. "One can say there was an inevitability to this crisis."

The drama appeared to spring forth without warning, even though the United States and Britain delivered a tough letter to Abbas a week ago warning they would immediately terminate the 2002 agreement to monitor the Jericho prison unless he took steps to improve security . . .

British Foreign Secretary Jack Straw told the House of Commons yesterday that the Israelis were notified the letter was sent to Abbas on March 8, as required under the 2002 agreement, although it was not publicly disclosed.

A senior State Department official said it appeared the letter to Abbas did not get the "high level of attention it deserved" from the Palestinians. But he said that Assistant Secretary of State C. David Welch raised the problem in a meeting with Abbas during a trip to the West Bank on Feb. 25.

Still, at least one U.S. official indicated Washington was surprised the British left their posts yesterday.

The British led the joint mission, which employed mostly retired military and policy officers, and the six U.S. monitors were not scheduled to be on duty yesterday. The senior administration official said Washington did not know the precise date the British would leave, but "we knew it was coming very soon."