US President Donald Trump said he had called off a secret summit with the Taliban and Afghanistan's leader, abruptly slamming the door on a year of diplomacy to end the US' longest war.

In a Saturday evening bombshell, Trump said that he had planned ­unprecedented, albeit separate, talks with the two sides on Sunday in Camp David, the presidential retreat in Maryland, but that the Taliban's persistent, grisly violence made them untrustworthy partners.

"Unbeknownst to almost everyone, the major Taliban leaders and, separately, the President of Afghanistan, were going to secretly meet with me at Camp David on Sunday," Trump said in a tweet.

"Unfortunately, in order to build false leverage, they admitted to an attack in Kabul that killed one of our great great soldiers, and 11 other people. I immediately cancelled the meeting and called off peace negotiations."

"What kind of people would kill so many in order to seemingly strengthen their bargaining position? They didn't, they only made it worse!" Trump said.

A US soldier and another service member from Romania were killed in the bombing on Thursday in Kabul - the latest major attack claimed by the Taliban even as they negotiated with a US envoy on the withdrawal of thousands of troops.

Trump would have met the Taliban at Camp David days before the 18th anniversary of the September 11 attacks, which triggered the US invasion that toppled the militants' regime.

Washington was jolted by the announcement from Trump, who is fond of dramatic gestures but whose Twitter pronouncements have often come into question later.

"Why a lethal attack in Kabul on Thursday would be the reason for calling it off, considering the multiple recent Taliban attacks, is unclear," said ­International Crisis Group's Asia director Laurel Miller, who earlier served as the US special representative on ­Afghanistan.

Congressman Tom Malinowski, a Democrat who has been pressing for clarity on the US strategy in Afghanistan, called the idea of Taliban leaders at Camp David "weird."

"But I'm glad the president called off this farce, and hope this good decision sticks," Malinowski tweeted.