WASHINGTON — Congress is raising questions about the secret U.S. diplomacy with Iran that led to last month's nuclear breakthrough.

Republican Sen. Bob Corker recently visited Oman, the site of several clandestine meetings between U.S. and Iranian officials. On Wednesday, he asked one such official to explain.

The official (Puneet Talwar), a White House adviser nominated for a senior State Department post, said he met Iranian officials in the summer of 2012 and in March of this year. He said the talks only heated up after Iran's moderate President Hassan Rouhani took office in August.

Sen. Marco Rubio made clear his disappointment at being left in the dark.

On Tuesday, another Republican, Rep. Matt Salmon, accused the State Department of deliberately misleading the public.