United States senator John Kerry says it would be "irresponsible" to send more US troops to Afghanistan amid a deepening election crisis that has placed the Kabul government's legitimacy at stake.
President Hamid Karzai's government has been plagued by uncertainty and questions of legitimacy after allegations of widespread fraud in the August elections, whose preliminary results put him on top.
Senator Kerry, the chair of the senate's foreign relations committee, said the United States should not proceed with a new Afghanistan strategy committing a potentially major increase in US resources, without first securing a clear partner in Kabul.
Two months after Afghans voted in controversial presidential elections, electoral authorities are due to announce whether a run-off poll is needed.
President Hamid Karzai leads preliminary results of the 20 August poll with about 55% of the vote. His main rival Abdullah Abdullah has about 28%.
To win the presidential election without a run-off a candidate needs 50% plus one vote.
The election has been overshadowed by allegations of fraud, mostly against Mr Karzai, including findings by EU observers that a quarter of all votes, or 1.5 million ballots, were suspicious.
For Mr Karzai's tally to fall below 50%, more than 250,000 votes would have to be thrown out from his tally.
Officials said Afghan politicians and their international backers have been involved in days of talks in the hope of averting a run-off.
An official announcement is to be made by the Independent Election Commission (IEC), widely regarded as pro-Karzai, which acts on the orders of the UN-approved Electoral Complaints Commission (ECC).