Afghan President Hamid Karzai Wednesday broke off crucial security talks with the United States, angry over the name given to a new Taliban office in Qatar that is meant to facilitate peace negotiations.

The ongoing Afghan-US talks must reach an agreement if Washington is to maintain soldiers in Afghanistan after a NATO combat mission ends next year.

Karzai's decision to suspend the talks threatens to wreck US efforts to start a dialogue with the Taliban, which President Barack Obama had welcomed as an important step towards ending 12 years of war.

The prospect of peace received a further reality check Wednesday when the Taliban claimed an overnight rocket attack that killed four US troops at the largest US-led military base in Afghanistan.

Explaining the suspension of the security talks, Karzai's spokesman Aimal Faizi told AFP: "There is a contradiction between what the US government says and what it does regarding Afghanistan peace talks.

"The president suspended the BSA (Bilateral Security Agreement) talks with the US this morning."

The row centres on the Taliban office using the title "Islamic Emirate Of Afghanistan" -- the formal name of the Islamist movement's government from 1996 until it was toppled in 2001.

"The president is not happy with the name of the office. We oppose the title the 'Islamic Emirate of Afghanistan' because such a thing doesn't exist," said a palace official who declined to be named. "The US was aware of the president's stance."

It was the latest incident to underline the troubled relations between Karzai and his American allies, which have degenerated into public spats in the past.

Many Afghans reacted with scepticism to news that the Taliban were opening an office in Qatar and could be ready to enter peace talks.

"It is a historic mistake by the US government, recognising and giving legitimacy to a terror network who are killing every single day Afghan civilians, women and children," Shukria Barakzai, a moderate female lawmaker told AFP.

Karzai has previously opposed direct Taliban-US talks, but had appeared to embrace the new office, which opened in Qatar on Tuesday.

He said he had ordered envoys to Qatar to try to negotiate with the Taliban as US officials said their own talks with the group could begin this week, though he stressed that any dialogue should move from Doha to Afghanistan as soon as possible.

On Tuesday NATO formally transferred responsibility for security to Afghan forces. About 100,000 foreign combat troops, 68,000 of them from the US, are due to withdraw by the end of next year.

But a continued American military presence is considered vital to support the US-backed Kabul government amid an ongoing Taliban insurgency.

In Iraq, where violence has surged, efforts to reach a similar arrangement collapsed when Baghdad refused to grant US soldiers immunity from prosecution.

Just hours before the Taliban's attack on the Bagram airbase, Obama welcomed the planned talks as an "important first step", although he warned of a bumpy road ahead.

The Taliban broke off contact with the Americans last year and have always refused to negotiate with Kabul. On Tuesday, they said their new office would "open dialogue between the Taliban and the world", but made no direct reference to peace talks.

A divided insurgency is also likely to complicate talks, amid doubts as to whether the Haqqani network of warlord Jalaluddin Haqqani, a former CIA asset turned Al-Qaeda ally, is ready to embrace dialogue.

A Taliban spokesman in Qatar, Mohammad Sohail Shaheen, confirmed before the Bagram strike that the armed group would continue to attack US targets in Afghanistan at the same time as holding any talks.

"There is no ceasefire (with the US) now. They are attacking us and we are attacking them," Shaheen told Al-Jazeera news channel, adding that the Taliban "simultaneously follows political and military options".

A US defence official confirmed that four Americans were killed in the rocket attack on the sprawling airbase north of Kabul.

Before the attack, Obama insisted the Taliban would have to renounce ties to Al-Qaeda, halt violence and commit to the protection of women and minorities. He warned that NATO forces remained "fully committed" to battling Al-Qaeda.

The Taliban were driven from power by US-backed rebels after the September 11, 2001 attacks. They have since mounted a guerrilla war against the Afghan government and maintain rear bases in Pakistan.

A US official said American and Taliban envoys would meet in Doha "in a couple of days", after which the Taliban would meet Afghan peace envoys.

In opening their mission, the Taliban did not explicitly renounce Al-Qaeda, which they refused to expel after the 9/11 attacks, but did vow to prevent attacks being launched from Afghanistan.