The United States and Afghanistan are moving forward with a deal to let US forces stay beyond 2014 to fight Al-Qaeda remnants, a US official said Monday, urging Kabul to stay focused to conclude the accord this month.

"We've made progress, but these kind of negotiations are complex with any country, as we know, from sort of the technical to the tough security issues," deputy State Department spokeswoman Marie Harf said.

"We always expected there would be sticking points and bumps in the road that needed to be resolved at a high level at some point in the process."

Afghan President Hamid Karzai said Monday that while the two sides were approaching a deal, he was still not satisfied.

Any accord had to provide Afghanistan with guarantees and preserve the interests of the Afghan people, he told a press conference.

Last week Karzai's spokesman said the deal was under threat because of disputes over US calls to be allowed to freely conduct military operations post 2014 in the country.

A further bone of contention was how the bilateral security agreement (BSA) should define an attack on Afghanistan that would trigger US protection.

"We believe that when terrorists are sent to commit suicide attacks here, that is also aggression," Karzai's spokesman Aimal Faizi said on Tuesday, referring to Pakistan-based militants whom Afghanistan believes are supported by Pakistani intelligence services.

"We are a strategic partner of the US and we must be protected against foreign aggression."

Washington was prepared to "conclude a reasonable" deal, Harf insisted.

"It's clear to us that the Afghan leadership is focused on making sure that they have an agreement that addresses the security needs of the Afghan people," she told reporters.

But she warned that after October it would become "a little bit more difficult" to reach an agreement, as Afghans would turn their attention to next April's presidential elections.

"We need to really be focused on this agreement and get it done soon," Harf said.

An accord also needed to be wrapped up to allow US and NATO planners to be able to go ahead with making arrangements for any future posture, she said.

The US aims to withdraw the bulk of its 57,000 troops in Afghanistan by the end of 2014, but it has tentative plans to retain some bases and a smaller force of around 10,000 after that.