US Secretary of State John Kerry on Sunday appeared to give Afghan President Hamid Karzai extra time to sign a bilateral security deal, saying the pact did not have to be concluded by January.

And while he said it must be signed as soon as possible, he raised the option for the first time that the deal governing the presence of US troops in Afghanistan beyond 2014 could even by inked by Karzai's successor, who will be chosen in April elections.

The United States wants the deal to ensure legal protection for any troops left behind in a training role after the international forces leave in late 2014. Washington has warned that, without a pact, it may pull all its military out.

In an interview with ABC television, the top US diplomat stressed the bilateral security agreement needed to be signed as soon a possible.

"If American forces were not there, I think there would be serious challenges with respect to Afghanistan's security," Kerry said.

"But ... I believe that Hamid Karzai, either he or his successor, will sign this," he said, before adding "I think he needs to sign this."

Asked whether Karzai had to sign the deal by January, as had been agreed when the two sides launched negotiations a year ago, Kerry replied "no."

He stressed, however, that there was a cut-off date, while refusing to go into specifics.

Washington has been infuriated by Karzai's sudden insistence after the details of the deal were agreed that the next Afghan president should sign it.

In a sign of the tensions, Kerry revealed he was talking to the mercurial Afghan leader through Karzai's minister and not directly.

"We have an agreement that's been negotiated and he has said to me personally, and as recently as a day ago, reiterated through his minister that the language is fine. He's not going... to seek a change in the language," Kerry said.

Upping the tensions, Karzai said Saturday during a visit to India that he no longer trusts the United States.

He also said he would not be "intimidated" into signing the pact.

"I don't trust them," Karzai said in an interview at an Indian hotel, accusing the US of saying one thing and doing another.

He referred specifically to a letter from US President Barack Obama assuring him that US forces would "respect" the safety of Afghans in their homes.

"They should prove it... implement the letter, respect Afghan homes," Karzai said.

"Just instruct no more bombings, and there will be no more bombings and, of course, launch the peace process publicly and officially," he said.

Kerry acknowledged the Afghan leader was seeking further guarantees.

"It's clear what I think he needs in terms of assurances. I believe it may be possible to try to move this forward," the top US diplomat told ABC.

He stressed again that the security pact needed to be in place so the United States and other NATO allies could start the planning for the 2014 withdrawal.

"You have more than 50 nations that have been involved in supporting this. They all have budgets. They all have planning requirements. And it is vital that Hamid Karzai recognize the importance of doing this," Kerry said.

"The US wants success in Afghanistan. And success means having an Afghan arms force that has the ability to sustain itself and provide security to the people of Afghanistan."