They said American spy agencies advised Bhutto's aides on how to boost security, although key suggestions appear to have gone unheeded.
Meanwhile, a Pakistani election official said the government would wait until Wednesday to declare a firm date for crucial elections scheduled for Jan. 8, but "it looks impossible" for the country to hold the vote as planned.
"After consulting representatives of the political parties, we will take any final decision about the date of polls tomorrow," commission spokesman Kanwar Dilshad said Tuesday
Sources in Islamabad told CBS News chief foreign correspondent Lara Logan on Monday that the vote woulddue to the widespread unrest following Bhutto's murder.
Opposition parties want the elections on time to capitalize on sympathy for Bhutto and anger at President Pervez Musharraf following her assassination. Western nations, which see the polls as key to restoring democracy in the Muslim nation, are also eager to see them take place as scheduled.
Former Prime Minister Nawaz Sharif, who has emerged as the country's most prominent opposition leader following the death of Bhutto, threatened street protests if the vote was delayed.
"We will agitate," he told The Associated Press. "We will not accept this postponement."
Senior U.S. diplomats had multiple conversations, including at least two private face-to-face meetings, with top members of Bhutto's Pakistan People's Party to discuss threats on the Pakistani opposition leader's life and review her security arrangements after a suicide bombing marred her initial return to Pakistan from exile in October, the officials told The Associated Press on Monday.
The intelligence was also shared with the Pakistani government, the officials said.
Much of what was passed on by the U.S. to Bhutto's security detail dealt with general threats from Taliban extremists and al Qaeda sympathizers and "was not actionable information."
The officials said Bhutto and her aides were concerned, particularly after the October attack, but were adamant that in the absence of a specific and credible threat there would be few, if any, changes to her campaign schedule ahead of parliamentary elections.
"She knew people were trying to assassinate her," said an intelligence official. "We don't hold information back on possible attacks on foreign leaders and foreign countries." The official added, however, that while the U.S. could share the information, "it's up to (the recipient) how they want to take action."
"We gave them a steady stream of intelligence," one official said.
The officials spoke to AP on condition of anonymity due to the sensitivity of the matter and amid widespread disbelief over the Pakistani government's assertion that Bhutto died not from bullet or shrapnel wounds but from injuries sustained while hitting her head on her vehicle's sunroof during Thursday's attack by a suicide bomber and gunman.
The dispute over the government's explanation of how she died intensified after a medical report did not state what had caused her injuries and a video obtained by Britain's Channel 4 television showed a man firing a pistol at Bhutto from just feet away as she poked her head out of the sunroof. In the footage, her hair and shawl jerk upward and she falls into the vehicle just before an explosion. No police are seen trying to push the crowd away.
The Bush administration has quietly joined calls for Pakistan to allow international experts to join the probe into Bhutto's Dec. 27 slaying. The officials said they expected an announcement soon that investigators from Britain's Scotland Yard would be asked to play a significant role. Any U.S. involvement would be limited and low-key, they said.
Pakistan's ambassador to the United States, Mahmud Ali Durrani, said Monday that his government would welcome outside experts to help investigate the assassination, according to The New York Times. But Durrani said his government would not endorse a separate, outside investigation.
"Pakistan is open to international expertise, international support and international help because it's in our interests," Durrani told the newspaper in a story posted late Monday on its Web site.
On Saturday, a Pakistani interior ministry spokesman, Javed Iqbal Cheema, defended his government's ability to carry out an investigation, saying, "I think we are capable of handling it."
In Islamabad, the government issued a statement Tuesday saying it was "committed to a thorough and transparent investigation and will not shy away from receiving assistance from outside, if needed."
In the meetings with U.S. officials, Bhutto aides did not ask the United States to help protect her but did inquire about the feasibility of hiring private U.S. or British bodyguards, an idea discouraged by the Americans who argued that a noticeable Western security detail would increase the threat and might become a target itself, the officials said.
Instead, the U.S. diplomats recommended as many as five reputable local Pakistani and regional firms that could be contracted to supplement Bhutto's security and urged the party to limit the size, scope and type of her public appearances, upgrade armoring on vehicles in which she might travel and require her to wear protective clothing, the officials said.
The officials said Zardari rejected using private Pakistani security companies due to fears they might be infiltrated by extremists even though several of the recommended companies have international components and are used by Western embassies to protect personnel.
Anne Tyrell, a spokeswoman for the private U.S. security company Blackwater Worldwide, known for its operations in Iraq, said her company had been approached about possibly providing protection for Bhutto, "but unfortunately, an agreement was never reached."
While Bhutto's staff did take some steps to improve the safety of the party's vehicles, the officials expressed surprise that the car in which she was riding when attacked had a sunroof and stressed that they would have strongly advised her against popping her head out of it in the presence of large numbers of people.
In addition to advising Bhutto's aides, as they worked to forge a political reconciliation and possible power-sharing deal between the opposition leader and Pakistani President Pervez Musharraf, the U.S. diplomats made numerous appeals to the Pakistani government to grant requests from her party to ensure Bhutto's safety, the officials said.
But some requests, such as those for advanced technology and massive police presences in outlying towns, either could not be met or were deemed unreasonable by the government, a position the United States reluctantly conceded, the officials said.
The State Department, meanwhile, angrily denied suggestions that U.S. officials had ignored or minimized the threat to Bhutto even as they were encouraging reconciliation between her and Musharraf.
"It is simply untrue and I simply do not understand why anyone, anywhere would assert that the United States did not have concerns, minimized those concerns, or was not very active in trying to ensure that she was provided with whatever kind of security support she required," deputy spokesman Tom Casey told reporters.
"We discussed those concerns regularly both with her and officials from her party and with President Musharraf and with his government," he said. "We always, in every instance, took those concerns seriously. We were very active in trying to ensure that any information we had that was relevant to her situation was passed on to her as well as those responsible for her security."