John Kerry says Afghanistan is at a critical moment of the transition.
US Secretary of State John Kerry is expected to visit Kabul for last-ditch efforts to mediate a solution to Afghanistan's escalating election crisis, Afghan officials said on Thursday.
The standoff between rival candidates Abdullah Abdullah and Ashraf Ghani over the disputed June 14 runoff vote has sparked concern that protests could escalate into violence - raising the spectre of the devastating civil war of the 1990s.
But the two camps voiced hope that Kerry could find a way out of the impasse, which comes at a crucial time with NATO's 50,000 combat troops pulling out, leaving Afghan forces to fight a resurgent Taliban insurgency.
Kerry, speaking on a visit to China on Thursday, said Afghanistan was at a "critical moment of the transition... essential to the future governance of the country".
Abdullah inflamed tensions on Tuesday by claiming victory in the election, blaming fraud for putting him behind in preliminary results which showed Ghani had taken 56.4% of the vote.
US officials have not confirmed Kerry's visit, which Abdullah announced on Tuesday, but officials from both candidates' camps said he was expected soon and they were hopeful his visit could bear fruit.
"He is coming here to solve the election deadlock, and I think it is a positive step," Ghani campaign spokesman Abdul Ali Mohammadi told AFP.
Abdullah's spokeswoman Lailuma Ahmadi also welcomed Kerry's visit.
"We think it is a positive move and we welcome any move and efforts to separate clean votes from unclean votes or fraudulent votes," Ahmadi said.
"We don't say they (the US) should meddle in the Afghan election process but they should help in the transparency of the election."
Kerry said he had been in touch with both candidates "several times", as well as outgoing President Hamid Karzai.
"We would encourage both of them to not raise expectations with their supporters, to publicly demonstrate respect for the audit process and the accountability process and also to show critical statesmanship and leadership at a time when Afghanistan obviously needs it," he said.
The vote base of Abdullah, an eye doctor, is among the Tajiks and other northern Afghan groups, while Ghani attracts much of his support from the Pashtun tribes of the south and east, ominously recalling the ethnic faultlines of the 1992-96 civil war.
Former World Bank economist Ghani, who says he won the election fairly, has called for the election timetable to be respected.
The final result is due out around July 24 after the adjudication of complaints and an anti-fraud audit.
US President Barack Obama stepped in earlier this week to warn the two rivals against "resorting to violent or extra-constitutional means" - a response to reports that Abdullah planned to announce a parallel government.
Abdullah's supporters took to the streets 10 days ago when reports emerged that Ghani was well ahead in the vote count, though protests have so far been peaceful.
Karzai, who is constitutionally barred from a third term in office, has stayed publicly neutral in the lengthy election, but Abdullah supporters accuse him of fixing the vote in Ghani's favour.
Highlighting the fragility of the security gains made in 13 years of fighting the Taliban, the UN on Wednesday revealed that civilian casualties in the conflict soared by 24% in the first half of 2014.
Ground combat is now causing more deaths and injuries than improvised explosive devices in a worrying sign of spreading conflict, the UN warned.