By Kylie Morris BBC correspondent in Kabul

The US special envoy to Afghanistan has urged all regional leaders to co-operate with the central government of Hamid Karzai.

Zalmay Khalilzad says the United States will judge them by that measure.

Dr Khalilzad has returned to the Afghan capital after meeting northern power brokers General Abdul Rashid Dostam and his rival, Ustad Atta Mohammed.

Dostam: Met Khalilzad

There have been fresh clashes in the past week between forces loyal to the pair and at least five people have been killed as a result.

The US envoy said both men had committed themselves to work for peace and stability and he is hopeful they will do so.

Patience running out

His comments follow tough talk from President Karzai, who over the weekend threatened to sack regional warlords who abused their power.

In a national address, he said his patience had finally run out with officials committing crimes in the name of the government.

Now the US administration has reiterated its support for the Afghan president's position.

The American special envoy says the development of the national army and police and a proper justice system is crucial to the reconstruction process.

Prisoners return

The US envoy was also able to confirm that three Afghans who had been held prisoner at the detention centre in Guantanamo Bay in Cuba have been returned to the American air base at Bagram, north of Kabul.

He says the three have been cleared of any links with al-Qaeda and so have been sent back.

He was unable to reveal whether they had received any compensation over their detention, but added they do get some assistance.

A local spokeswoman for the International Committee of the Red Cross says the three were handed over to local authorities.