A Republican senator is blocking President Barack Obama's nominee for ambassador to Russia over concerns that the U.S. might provide Moscow with sensitive missile defense information, AP reports.

The administration says it has no current plans to provide such data. It says, however, that the assurances sought by Illinois Republican Sen. Mark Kirk are so broad, they would prevent any substantial cooperation on missile defense.

The dispute with Kirk further complicates the Obama administration's efforts to get U.S.-Russian relations back on track at a time of increased tensions over missile defense.

Kirk is holding up the nomination of Michael McFaul, a senior adviser to Obama on Russia.