U.S. Congressional sources tell RFE/RL that approval of President Barack Obama's nominee for ambassador to Russia is being blocked by a senator from the opposition Republican Party.
The nomination of Michael McFaul, who served on Obama's National Security Committee and was his top Russia adviser, was approved by the Senate Foreign Relations Committee on November 29.
But according to a source in Congress who requested anonymity, Republican Senator Mark Kirk, from the state of Illinois, has placed what's known as a "hold" on McFaul's nomination.
The lawmaker is reported among those concerned that the Obama administration is planning to share classified missile defense data with Moscow.
A full vote by the Senate to confirm McFaul is now indefinitely delayed.
McFaul had previously had the support of both Obama's Democratic Party and the Republicans. But some Republicans who disagree with Obama's "reset" policy with Moscow are now objecting to McFaul's appointment.
Obama has the option of appointing McFaul ambassador without the Senate's approval in what's known as a "recess appointment," when Congress is not in session later this month.
During a Congressional recess in December 2010, Obama bypassed senators' holds to appoint Matthew Bryza as the U.S. envoy to Azerbaijan.