The United States and Afghanistan launched crucial talks Thursday on the status of US forces remaining in Afghanistan after the NATO withdrawal of combat troops in 2014.

A key element of any agreement will be the question of immunity for US troops from prosecution in local courts, but this was not discussed in the first round of talks, negotiators said.

In Iraq, Washington pulled out all of its troops after failing to get Baghdad to grant its soldiers immunity, and President Hamid Karzai has warned there could be similar problems in Afghanistan.

The issue has been highlighted by the massacre of 16 villagers earlier this year, allegedly by a rogue US soldier who was flown out of the country and is facing hearings in the United States.

The number of troops who will stay in the country and their roles in the fight against insurgents led by Taliban Islamists are also unlikely to be dealt with in the early rounds of talks, according to sources close to the negotiations.

"We were very encouraged by today's round that we could speak frankly with each other," US chief negotiator and deputy special representative to Afghanistan and Pakistan, James Warlick, told a news conference.

"And I am very confident that the rounds ahead will lead to the conclusion of a document which is in both of our countries' interest.

"This document is intended to provide legal authorities for United States armed forces and its civilians component to continue a presence in Afghanistan with full approval of the government," he said.

Afghanistan's chief negotiator and ambassador to Washington, Eklil Hakimi, said the security agreement was one of the most important elements of the long-term strategic partnership deal already signed with Washington.

"In this agreement, the quantity, quality, defence cooperation and security presence of the US in Afghanistan after... 2014 are included," he said.

The negotiations would be based on Afghanistan's national interests and sovereignty and ensuring peace and stability, strengthening democracy and the capability of Afghan armed forces, Hakimi said.

Both men said the deal, which could take a year to negotiate, would pose no threat to any other country in the region.

Iran, in particular among Afghanistan's near neighbours which include China, Pakistan, India and former Soviet Central Asian states, has made clear its objection to any military deal between Washington and Kabul.

The US has stressed that it is not seeking permanent bases in Afghanistan. It is also considered likely to shy away from a security guarantee, which would require it to come to the nation's assistance against aggressors.

That, however, is seen as one of the targets of Afghan negotiators.

US President Barack Obama flew to Kabul to sign a Strategic Partnership Agreement with Karzai in May, but it did not cover the legal status of any troops remaining behind after the 2014 withdrawal of NATO combat forces.

The US has some 68,000 troops in the NATO force of more than 100,000 and is expected to leave an as yet unspecified number to help train, advise and assist Afghan forces in the war against Taliban insurgents.

US military officers have said they envisage a follow-on force of around 15,000 personnel, but the number, and their role, is expected to be announced soon by the new administration of US President Barack Obama.

The negotiations come against a background of relations strained by perceived atrocities by US troops and an increasing number of so-called green-on-blue attacks, in which Afghan forces turn their weapons against their NATO allies.