LONDON (REUTERS) - United States (US) Secretary of State John Kerry said on Monday complaints against Afghans working for US special forces in Afghanistan would be investigated, a day after Afghan President Hamid Karzai ordered US troops to leave a critical battleground province.

Mr Karzai's spokesman on Sunday said Mr Karzai had decided that all US special forces must leave Wardak Province within two weeks, after accusations that Afghans working for them had tortured and killed innocent people.

The move could further complicate talks between the US and Afghanistan over the presence of American troops in the country once most North Atlantic Treaty Organisation (Nato) forces leaveby the end of 2014.

"With respect to Afghanistan and Wardak province, I understand the concerns that they have expressed. And appropriately, any complaints that they may have ought to be appropriately evaluated, and they will be, I can assure you," Mr Kerry told reporters during a visit to London.

Mr Kerry said it was up to the Nato-led International Security Assistance Force (ISAF), to investigate.

On Sunday, ISAF said it was aware of the allegations of misconduct, but would not comment further until it had spoken to Afghan officials.

Relations between Mr Karzai and his international backers have at times been fraught, with the Afghan president warning that civilian deaths could sap support for foreign troops and fuel the insurgency.

Earlier this month, Mr Karzai said Afghan security forces would be banned from calling for Nato air strikes in residential areas after 10 civilians died in one such strike.

Mr Karzai's anger over the conduct of Afghan troops working with ISAF raises the pressure on Afghan forces as they increasingly assume control of security.

Nato and its partners are racing against the clock to train Afghanistan's 350,000-strong security forces, though questions remain over how well the Afghans will be able tackle the insurgency in the face of intensifying violence.

Mr Kerry said Mr Karzai had "many legitimate evaluations" of where things have wrong or could be improved, but indicated that negotiations on transition and an agreement on the presence of some US troops in Afghanistan after 2014 were going well.

"We're working on a bilateral security arrangement; we're working on this transition process. We've had a very good conversation with the President (Karzai) in the last days," Mr Kerry said.