A senior adviser to Yasser Arafat (searchon Wednesday accused the United States of "blackmailing" the Palestinians (search) by withholding aid -- unusually harsh criticism reflecting exasperation with perceived U.S. disengagement from the Mideast conflict.

Israeli (search) and Palestinian officials, meanwhile, failed again Wednesday to reach agreement on a date for a meeting between their two leaders. Such a summit is seen as vital for reviving the U.S.-backed "road map" peace plan.

Israeli Prime Minister Ariel Sharon (searchhas said repeatedly in recent weeks he would go ahead with unilateral measures, including dismantling some settlements and imposing a boundary on the Palestinians, if there is no progress in coming months.

Earlier this week, he proposed dismantling virtually all the Israeli settlements in the Gaza Strip, a political bombshell that is threatening his center-right coalition.

Sharon is facing mounting opposition, including from within his Likud Party. Challenging his critics, Sharon said Wednesday he would support a referendum on the issue. Polls indicate a large majority of Israelis support a Gaza pullout.

State Department spokesman Richard Boucher offered a mixed response to Sharon's Gaza initiative. Boucher said that while removing settlements is good, the United States is concerned about "any steps that could attempt to unilaterally end the process or unilaterally impose a settlement."

In the West Bank city of Ramallah, Jibril Rajoub, a senior security adviser to Arafat, charged that the United States is cutting aid and scaling back involvement in Mideast diplomacy because the Palestinians have failed to find those responsible for an attack on a U.S. convoy in Gaza in October. Three U.S. security guards were killed in the roadside bombing.

"I think this is blackmail," said Rajoub, who enjoyed close relations with U.S. agencies when he served as security chief in the West Bank.

U.S. officials have complained about lack of progress in the investigation of the attack. In December, U.S. envoy William Burns said the United States "cannot carry out our full range of assistance projects" without progress in the probe.

Boucher on Wednesday dismissed Rajoub's comments as "ridiculous." He said Palestinians should improve their security, but added that the United States remains engaged in peacemaking.

Boucher also said Israeli Vice Premier Ehud Olmert will confer Thursday with Secretary of State Colin Powell in Washington. Olmert is Sharon's top deputy.

The United States offered a give up to $5 million to anyone who provides information that leads to the "conviction or arrest" of those responsible for the convoy attack.

"To bring justice for those responsible for this attack, the U.S. government is offering a reward of up to $5 million, plus protection for an informant's identity and relocation with their families, for information leading to the arrest or conviction of individuals who committed or aided this attack," a statement from the U.S. Embassy in Tel Aviv said Thursday.

Palestinian officials have complained privately about what they see as Washington's disengagement from the Israeli-Palestinian conflict during a U.S. election year. However, a harsh public comment like Rajoub's is rare.

The road map, meanwhile, suffered another setback Wednesday, when aides for Sharon and Palestinian Prime Minister Ahmed Qureia failed to agree on a summit agenda. Palestinian negotiator Saeb Erekat said he hoped a summit could be arranged after the next meeting of aides, which would take place "in a few days."

Qureia has welcomed Sharon's proposal to dismantle Gaza settlements, but Palestinian officials say a summit must yield immediate results, such as an easing of travel restrictions on Palestinians.

Sharon is coming under increasing pressure from hawkish legislators to abandon the Gaza plan. Ten Likud legislators signed a letter Wednesday, saying they opposed a removal of settlements.

Others demanded he put the program to a referendum. On Wednesday, Sharon took up the challenge and embraced the referendum idea.

A senior Israeli official, speaking on condition of anonymity, said Sharon would welcome a referendum because the issue cuts across ideological lines and "has overwhelming support among the public."

Israel has never had such a referendum. A plebiscite is required over any treaty that changes Israel's borders, but that would not apply here.

Sharon's center-right coalition government was not likely to survive removal of settlements, with two pro-settler parties poised to resign. Sharon said he would call elections if necessary.

Rajoub was suspicious of Sharon's intentions. "I hope Mr. Sharon is not bluffing us or playing such a dirty game to ... avoid the road map," he told reporters.

Politicians and analysts questioned whether Sharon, the historical champion of the settlements, actually planned to remove any of them. Some said he might be trying to deflect attention from a widening bribery scandal that could ensnare him.

Sharon is to face police questioning on Thursday after a real estate developer was indicted for allegedly bribing Sharon over a business deal. If indicted, Sharon would probably have to suspend himself from office. Sharon has denied wrongdoing.