U.S. Secretary of State John Kerry on Thursday arrived in Kabul on an unannounced visit to mediate the ongoing crisis in Afghanistan between the presidential candidates and current President Hamid Karzai.

The transition of power in Afghanistan has been up in the air after Abdullah Abdullah on Monday rejected - claiming fraud - the preliminary results of the presidential elections that placed Ashraf Gani out front in the balloting.

Kerry will reinforce President Barack Obama's "message that we expect a thorough review of all reasonable allegations of fraud and that we will not accept any extra-constitutional measures," State Department press office director Jeff Rathke said in a statement.

Both Obama and Kerry have warned Afghanistan that Washington will suspend financial and security aid if it is determined that the winner of the presidential vote came to power illegitimately.

"While the United States does not support an individual candidate, we do support a credible, transparent, and inclusive process that affirms the Afghan people's commitment to democracy, and that produces a president who can bring Afghanistan together and govern effectively," the statement added.

Kerry traveled to Kabul from Beijing and his visit was neither scheduled nor confirmed until now by the State Department, although several Afghan media outlets and Abdullah had announced that the secretary would visit the country.

The campaign teams of Abdullah and Gani, who faced off in the second electoral round on June 14, have emphasized that Kerry's role will only be that of an "adviser" in the negotiations to find a peaceful way out of the crisis.

It is foreseeable that Kerry will also try to get the stamp of approval from both candidates on the bilateral security accord for the postwar period, which Karzai refused to sign but which in principle has the support of the two presidential hopefuls.

The NATO force in Afghanistan will conclude its mission in the country at the end of this year, but the United States has announced that it will maintain some 9,800 soldiers there until the end of 2016, a plan which requires a green light for the security pact from Kabul. EFE