The United States and the Taliban raised hopes for a negotiated peace in Afghanistan with commitments to meet this week after 12 years of bloody and costly war between American-led forces and the insurgents.

The Taliban opened an office in Doha, the Qatari capital, on Tuesday to help restart talks and said it wanted a political solution that would bring about a just government and end foreign occupation of Afghanistan.

US officials said the talks would start in Doha on Thursday but cautioned that the on-again, off-again peace process would likely be messy and has no guarantee of success.

"It's going to be a long, hard process if indeed it advances significantly at all," a senior US official said.

The Afghan government of President Hamid Karzai also said it was sending a team to Doha and a senior official said the Taliban was willing to consider talks. But the insurgents made no immediate comment on the claim.

The ultimate goal of the diplomatic manoeuvring is to get representatives of the Afghan government and the Taliban into direct negotiations on the country's future. The Taliban have thus far refused such talks, calling Karzai and his government puppets of the West.

Nonetheless, the diplomatic announcements represented the first signs of optimism in Afghan peace efforts for many months, and come as the US-led war effort reaches a critical juncture. The NATO command in Kabul on Tuesday completed handing over lead security responsibility to Afghan government forces across the country.

NATO plans to end all combat operations in Afghanistan by December 2014.

President Barack Obama said US combat operations would not cease and, in a reminder that the insurgents continue to fight, four US troops were killed in an attack on Bagram Air Force Base in Afghanistan, a US official said later on Tuesday.

Obama, travelling in Europe, cautioned against expectations of quick progress, saying the peace process would not be easy or quick.

"This is an important first step towards reconciliation; although it's a very early step," Obama said after a G8 meeting in Enniskillen, Northern Ireland. "We anticipate there will be a lot of bumps in the road."

Karzai said his government would send a team to Qatar but added the talks should quickly be moved to Afghanistan.

"We hope that our brothers the Taliban also understand that the process will move to our country soon," he said.

It was not immediately clear why the Taliban had agreed to resume talks with the United States, which they broke off in March 2012. The question of entering negotiations has caused rifts between Pakistan-based Taliban senior leaders and younger battlefield commanders, who oppose the move, US officials have said.

In opening the Qatar office, the Taliban said it was seeking a political solution, but added that no dates had been agreed for talks. Taliban representative Mohammed Naeem told a news briefing in Doha that the group wanted good relations with "all of the world countries."

"But the Islamic emirate (Taliban) sees the independence of the nation from the current occupation as a national and religious obligation," he said.

US officials said that in the talks in Doha, the United States would stick to its insistence that the Taliban break ties with al Qaeda, end violence, and accept the Afghan constitution, including protection for women and minorities.

The Taliban is expected to demand the return of former senior commanders now detained at the US military prison at Guantanamo Bay in Cuba - a move many in the US Congress oppose - as well as the departure of all foreign troops.

But the United States hopes to keep a force, of as yet undetermined size, in the country after the end of the NATO combat mission.

The peace negotiations also face sceptics in the US Congress.

"Until the Taliban confirm, not just in words but in action, that they have renounced all terrorist activity and support, we should not reward them by participating in any reconciliation efforts," Senator Saxby Chambliss, a Georgia Republican, said in a statement.

US officials said the initial meeting with the Taliban was expected to involve an exchange of agendas, followed by another meeting a week or two later to discuss next steps.

A US official said he expected the initial meeting would be followed within days by another between the Taliban and the High Peace Council, a structure set up by Karzai to represent Afghanistan in such talks.

The US officials, who spoke on condition of anonymity, said the level of trust between the Afghan government and the Taliban remained low, and played down expectations that the talks would quickly lead to peace.

"We need to be realistic," said one official. "This is a new development, a potentially significant development. But peace is not at hand."

A senior US official said Pakistan, which has provided sanctuary to the Taliban despite its professed support for the battle against Islamist militancy, had recently been supportive of the peace process.

"There has in the past been scepticism about their support, but in recent months I think we've seen evidence that there is genuine support and that they've employed their influence such as it is to encourage the Taliban to engage," he said.

A US official said the talks would be conducted on the Taliban side by its political commission, with the authorisation of its leader, Mullah Omar. The main US interlocutor has been Tayeb Agha, whom Washington considers close to Omar.

James Dobbins, the new special representative for Afghanistan and Pakistan, will lead the US side.

Also represented will be the Haqqani network, considered the United States' deadliest foe in Afghanistan. The top US and NATO commander in the country cast doubt on Tuesday over whether it could make peace.

"All I've seen of the Haqqani would make it hard for me to believe they were reconcilable," US General Joseph Dunford told reporters by phone from Kabul.-Reuters