Secretary of State John Kerry, in a conversation with his Russian counterpart Sergei Lavrov, raised "concerns" about an Iranian leader's recent travel to Moscow in violation of U.N. sanctions, according to the State Department. 

The travel by Maj. Gen. Qassem Soleimani, the shadowy Iranian Quds Force leader, was first reported by Fox News

Kerry "raised his concerns about the travel to Moscow" in a conversation with the Russian foreign minister, the State Department said in a statement Thursday. 

Soleimani's visit to Moscow came just 10 days after the comprehensive nuclear agreement was struck between Iran and six world powers, including the United States, in Vienna on July 14. 

Kerry, though, assured the Senate Armed Services Committee July 29 that "under the United States' initiative, Qassem Soleimani will never be relieved of any sanctions." Existing sanctions bar his travel from Iran. 

Soleimani and his Iranian-backed Shia militias are responsible for the deaths of hundreds of American soldiers in Iraq, according to senior military leaders. 

In 2011, the U.S. Treasury Department implicated Soleimani for his role in the failed assassination attempt of Saudi Arabia's ambassador to the United States at a popular Georgetown restaurant in Washington, D.C. 

The Quds Force is the international special operations wing of Iran's Revolutionary Guard Corps blamed for supporting terrorist activity throughout the Middle East. It has supported Iranian proxies in Lebanon, Syria, Iraq and Yemen. They are also supporting Shia militias against the Islamic State in Iraq, through some of the same units that once targeted American soldiers. 

In an increasing sign of a growing alliance, Iran’s semi-official Fars News Agency announced this week the start of Russian-Iranian naval exercises in the Caspian Sea.

On Wednesday, State Department spokesman Mark Toner confirmed Soleimani's travel after questioning from Fox News and the Associated Press. 

But despite that confirmation, another State Department spokesman appeared to backtrack Thursday afternoon at the next press briefing. 

"If the travel happened -- and I'm not in a position to confirm it -- if it happened, it would be a violation of Security Council resolutions, and certainly then a matter of serious concern to the United States," said spokesman John Kirby. 

The department's subsequent statement on the phone call with Lavrov appeared to, again, confirm the travel.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews