WASHINGTON – President Donald Trump said Saturday he canceled a secret Camp David meeting with Taliban leaders and is suspending Afghanistan peace negotiations after the group claimed responsibility for a car bomb this week that killed an American and 11 others.

The president said that Taliban leaders and Afghan President Ashraf Ghani were preparing to travel to the United States this weekend, presumably to finalize an agreement that has been in the works for months to reduce U.S. forces in Afghanistan. Instead, Trump said he abruptly canceled the meeting following the Taliban attack.

"I immediately cancelled the meeting and called off peace negotiations," Trump wrote in a tweet Saturday.“What kind of people would kill so many in order to seemingly strengthen their bargaining position?”

Trump disclosed that he had planned to meet with Taliban and Afghan officials on Sunday at Camp David, near the 18-year anniversary of the Sept. 11 attacks. Trump instead accused Taliban officials of trying to "build false leverage" ahead of those talks with the bombing.

Trump has said he wants to pull thousands of U.S. troops out of Afghanistan. His administration has been negotiating with the Taliban to reduce the roughly 14,000 troops now in Afghanistan. A U.S. envoy said on Monday that an initial agreement had been reached to achieve that goal, but Trump remained noncommittal about his support.

More than 2,400 American soldiers have been killed in the war, according to the most recent figures from the Pentagon.

As recently as Thursday, Secretary of State Mike Pompeo said the U.S. had “delivered” on its promises in Afghanistan and seemed to defend the emerging agreement.

“If you go back and look at the days following 9/11, the objectives set out were pretty clear: to go defeat al-Qaida, the group that had launched the attack on the United States of America from Afghanistan,” Pompeo said in an interview with a conservative website. “And today, al-Qaida … doesn’t even amount to a shadow of its former self in Afghanistan … We have delivered.”

The State Department referred questions about Trump's announcement to the White House.

Trump's desire to reduce America's military presence in Afghanistan has been fraught with political and military peril. Critics – including some of Trump's strongest supporters – fear a U.S. withdrawal will open the door for a resurgence of al-Qaida, as well as other terrorist groups operating in Afghanistan, such as the Islamic State.

The president is also under pressure to avoid a hasty agreement in order to achieve his campaign promise to reduce troop levels. That pressure has come from Afghan leaders, some Republican members of Congress and also hawks within his White House.

"This will make President Ghani happy but will slow down hopes for an early September deal," said Aaron David Miller, a diplomat who has worked for administrations of both parties. "Bottom line – there are no good withdrawal deals. If Trump wants out, (it's) likely a choice between bad and worse.”

Two NATO services members, including an American, were among a dozen people killed in the attack in Kabul on Thursday.

Brett Bruen, a former foreign service officer and global engagement director for President Barack Obama, expressed shock that any American president would invite the Taliban, a militant Islamic group that has targeted American soldiers throughout the war, to the U.S.

"Coming to the United States, let alone to a presidential retreat, is a prize saved for when real concessions have been made," Bruen told USA TODAY. "As we have seen in North Korea, Trump’s negotiations with our adversaries are marked by their preference for style over strategy. Ultimately it is a recipe for damaging our influence and the prospects for peace."

Trump's assertion that the car bombing prompted him to cut off talks seemed odd, given that the Taliban have been engaging in such violent attacks all through the current negotiations. Miller suggested there may have been more at play than Thursday's attack.

“It’s clear he’s coming under pressure from Pompeo and (National Security Adviser John) Bolton to avoid a hasty agreement that has U.S. making most of the concessions and the Taliban very few," he said.

Ned Price, a National Security Council spokesman for Obama, said Trump's actions are likely to undercut the negotiating process.

“What we heard from Trump today was his attempt to subvert and sensationalize the process, taking over for political gain a true diplomatic process that, by all accounts, had made genuine progress,” Price said.

“All of those diplomatic gains may now be in jeopardy,” he added.

Military leaders have resisted Trump's push for a full, speedy withdrawal. When asked about the potential peace deal last week, Marine Gen. Joe Dunford, chairman of the Joint Chiefs of Staff, told reporters that he’s "not using the withdrawal word."

There are about 22,000 coalition troops in Afghanistan now, 14,000 of them Americans.

Dunford stressed that an agreement with the Taliban would be "conditions based," a term favored by commanders who oppose deadlines for the drawdown of troops.

The emerging peace agreement also leaves major questions unresolved, most notably what role the Afghan government, which is backed by the U.S., will play in the future of the country. The Taliban had so far refused to negotiate with Ghani, demanding the complete withdrawal of all foreign military forces first.

It's not clear if those two parties could come to any power-sharing agreement. The Taliban already controls a large swath of the country, and the Afghan government remains heavily dependent on the U.S. military for its security, according to a recent analysis by Robert Pape, director of the Chicago Project on Security and Threats.

Asked about the emerging agreement on Wednesday, Trump declined to commit.

"We’re talking to the Taliban. We’re talking to the government. We’ll see if we can do something," Trump told reporters at the White House. "We have great soldiers. But they’re not acting as soldiers. They’re acting as policemen and that’s not their job."

Contributing: Tom Vanden Brook