WASHINGTON The US State Department has expressed concern over Kabul's decision to allow warlord Abdul Rashid Dostum to return home and participate in the electoral process.

'We have made clear to the government of Afghanistan our serious concerns regarding the return of Mr Dostum and any prospective role in today's Afghanistan,' Assistant Secretary for Public Affairs P.J. Crowley told a briefing in Washington.

He said that President Barack Obama had already asked his national security team to gather further information on Gen Dostum's background, including concerns that he might have been involved in the deaths of a significant number of Taliban prisoners of war a few years ago.

The White House team, he said, was continuing to gather that information.

The Obama administration also said that it had counselled the Karzai government against allowing the return of Gen Dostum, whose reputation threatens to cast a pall over the country's critical election process.

The controversial but still powerful Afghan warlord flew home from Turkey on Monday and immediately lent support to President Karzai, which could be critical in the multi-candidate presidential race.