US Secretary of State John Kerry is visiting Afghanistan to try to help resolve tensions over the country’s disputed presidential elections.
He will meet both of the candidates claiming victory in June’s run-off, Abdullah Abdullah and Ashraf Ghani.
Mr Kerry earlier warned against a power grab, amid reports that Mr Abdullah was planning a “parallel government”.
President Hamid Karzai is stepping down after more than 10 years in power after the US-led overthrow of the Taliban.
The fact Mr Kerry made this trip at such short notice, is an indication of just how concerned the US is about the risk of Afghanistan descending into post-election turmoil, the BBC’s Aleem Maqbool in Washington reports.
It has become clear that seeing the way Iraq has unravelled so dramatically after US troops’ withdrawal has made the White House all the more desperate that Afghanistan should fare better, our correspondent says.
‘Testing the limits’
Mr Kerry is also due to hold talks with the current president in Kabul.
America’s top diplomat earlier warned that Afghanistan risked losing security and aid support if anyone tried to “take power by extra-legal means”.
Preliminary results announced by Afghanistan’s election officials give Mr Ghani 56.44% of votes in the 14 June run-off.
Mr Abdullah, who fell just short of an outright majority in the first round, has 43.56%.
Both candidates have alleged fraud in the election. Votes are being re-checked at more than 7,000 polling stations – nearly a third of the total number.
Correspondents say recounts could significantly alter the final result, due on 22 July.
The UN Assistance Mission to Afghanistan has warned it will be “premature” for either side to claim victory.
It called on the presidential candidates to exercise restraint and prevent their supporters from “taking steps that could lead to civil disorder and instability”.
There are also concerns about a further deterioration in the security situation.
Taliban militants have been testing the limits of the Afghan army in recent weeks, with a major offensive in the southern province of Helmand, says the BBC’s Karen Allen in Kabul.
The withdrawal of foreign troops by the end of this year will be the litmus test of whether more than a decade of training and investment in building up Afghanistan’s own security forces has paid off, she adds.
President Barack Obama has said the US remained committed to Afghanistan provided the incoming president signed a security agreement.
Both Mr Abdullah and Mr Ghani have said they are committed to signing the deal with the US that would allow a small force to stay on.