UPDATE (3p.m)— Sources close to the Boston 2024 proposal confirm that the USOC will abandon plans to submit Boston as the host city for the 2024 Summer Games. Developing...

Under pressure from the US Olympics Committee to sign a host city agreement, Boston Mayor Marty Walsh said Monday he could not at this time sign a taxpayer guarantee because the risks of hosting the games are still not fully known.

The mayor's decision, along with Gov. Charlie Baker's continued wait-and-see approach to the games, could potentially contribute to the demise of the city's bid to host the 2024 Summer Games.

"This is me letting the taxpayers of Boston know where we stand on the Olympic bid," Walsh told reporters at a mid-morning press conference. He said he had "made it perfectly clear" he would not sign an agreement that put taxpayers' money at risk.

Walsh said it would be up to the U.S. Olympic Committee whether the bid moves forward even without a taxpayer guarantee.

According to a Baker aide, the U.S. Olympic Committee asked Baker for an "update" during a Monday morning conference call and the governor said he was waiting for a report on the Olympics bid from a consultant hired by the state. The report from The Brattle Group is due in mid-August.

Baker spokesman Tim Buckley said the USOC did not give any indication during the "short call" that it planned to pull its support for Boston or reaffirm the city as its choice to bid for the games.

"Even though we're only a few months into the Olympic process, I'm being asked to commit to signing the guarantee of the host city contract - language of which ... will not be released until September," Walsh said.

Walsh said he "always anticipated" being able to negotiate the contract with the International Olympic Committee. He said, "If committing to signing a guarantee today is what's required to move forward, Boston is no longer pursuing the 2024 Olympic and Paralympic Games."

Walsh, who has been the most high-profile political supporter of Boston's Olympic bid, said he had spoken to USOC CEO Scott Blackmun about his stance and Blackmun asked "if I could think about what I would say here" at the press conference, Walsh said.

"I don't think we're doing anything to damage the bid. We're talking the truth," Walsh said. Asked if his decision spelled the end for the Boston bid, Walsh said, "You'll have to ask to the USOC that."

Maintaining that there is still great appeal to hosting the international sporting event in Boston, Walsh appeared to move past that potentiality, saying, "I look forward to continuing these conversations as we move forward with Imagine Boston 2030" - a city-led planning process.

Walsh said the Olympic bid process had highlighted the potential for transformative development at Widett Circle, an industrial area outside of downtown.

"There's a potential there now regardless of what happens with the Olympics," Walsh said, suggesting efforts to build housing at Columbia Point and refurbish Harambee Park could also move forward separate from the games. He said he would not regret the Olympic process even if Boston is dropped by the USOC.

While Walsh was an early proponent, the Olympics effort in Boston was led by Boston 2024, a non-profit led by Richard Davey, the state's former transportation secretary, and Stephen Pagliuca, a businessman who ran for U.S. Senate five years ago.

Reports of U.S. Olympics officials looking toward Los Angeles as a potential alternative as well as reports of a pending ultimatum from the USOC in recent days appeared to weigh on Walsh.

"I'm hearing too many other voices talking about L.A.," Walsh said.

Asked if he expected on Monday to hear a final verdict on whether the USOC would stick with Boston, Walsh said, "According to what I read in the paper today, I guess so."

Cities have until Sept. 15 to declare to the International Olympic Committee their intention of whether to pursue the 2024 games.

Walsh said he would "absolutely never" put taxpayers on the hook for any liabilities associated with hosting the games and said he thought privately financed insurance policies could be sufficient to provide Olympics organizers with the requisite protections.

Citizens for a Say & Tank Taxes for Olympics, two groups pushing a ballot initiative aimed at preventing the use of tax funds to pay for the Olympics, said Walsh's announcement "smacks of a staged crisis."

"Mayor Walsh was never in a position to sign a guarantee, because of the scope of the Olympic bid financially and geographically," the groups said. "As before the press conference the decision is up to the Governor, as the state would have to be the one on the hook for the guarantee for the Olympics."

Matt Murphy contributed reporting.