By Jonathan Beale
BBC News, Washington
The US has signalled it could withhold aid to Uzbekistan if it continues to snub calls for an international inquiry into a bloody crackdown in Andijan.
The Andijan killings have been condemned by the UN
A United Nations report concluded that grave human rights violations were committed when Uzbek troops opened fire on demonstrators in Andijan in May.
The US has relied on Uzbekistan for military co-operation to carry out missions in neighbouring Afghanistan.
But that relationship is now coming under strain.
The United States has already withheld $8m of aid to Uzbekistan in protest at President Islam Karimov's record on human rights.
But the Pentagon has been anxious to maintain strong ties in order to continue using the Uzbek air base.
However, the US State Department is stepping up the pressure since the killings in Andijan.
It is still to decide whether to release a further $22m in aid for this year.
A State Department spokesman made clear that the handover of that money would largely depend on whether Uzbekistan agreed to an international inquiry.
President Karimov has so far rejected those calls, despite UN estimates that between 176 and 700 people died when troops opened fire on protestors.
The Uzbek government says the protesters were Islamic militants trying to seize power, though eyewitnesses dispute this.
Relations between Washington and Tashkent have noticeably cooled, with President Karimov turning to former cold war allies.
Russia and China have joined in calls for the US to set a date for the withdrawal of troops and planes from Uzbekistan.