The Afghan army Thursday took over extra responsibilities for security in the capital from NATO-led forces, but authorities kept the details under wraps on the largely symbolic move.

The Afghan defence ministry had announced its army would take responsibility for Kabul on August 28 in the first stage of a process that would eventually see international troops relinquish command of the entire country.

But on Thursday both sides would not give details, saying only some documents had been signed between them about a transfer of some authority.

"Some level of responsibility has been passed," said Brigadier General Richard Blanchette, spokesman for NATO's International Security Assistance Force.

"It's a continual process and it is something that will show how much improvement there has been on the Afghan national security forces' side," the Canadian officer told AFP.

The Afghan defence ministry would not comment. A defence ministry official confirmed, on condition of anonymity, that documents had been signed but said details would not be released for "security reasons."

The transition is mostly symbolic and will make little difference on the streets of Kabul, where Afghan police and soldiers already have a strong presence.

Authorities have stepped up security in the capital in recent months after a series of bombings and threats by Taliban-led insurgents trying to drive out the Western-backed government and its allies.

US-led forces invaded Afghanistan in late 2001 and ousted the Taliban regime for harbouring Al-Qaeda.

Nearly 70,000 international troops are helping the government fight the growing insurgent threat and rebuild its security forces. The Afghan army already takes the lead in some joint operations.