The United States said it would raise the issue of a recent Moscow visit by a senior Iranian general at the United Nations, saying it violated UN resolutions on Iran.
General Qassem Suleimani's visit to Russia, reportedly in late July, had not received much attention until it was mentioned in Iranian media and on America's Fox News channel in the context of the war in Syria.
US State Department deputy spokesman Mark Toner confirmed the trip Wednesday.
"So we've raised this travel with senior Russian foreign ministry officials, and we're going to raise it and address it further in New York," Toner said.
Suleimani, the Revolutionary Guard's commander of foreign operations, is one of several Iranian officials targeted by a 2007 UN travel ban because of their alleged links to Iran's nuclear or ballistic missile programs.
Despite the recent deal struck by Iran and world powers on its nuclear programme the sanctions against Suleimani remain in effect, Toner noted, adding the US would work to ensure "that there's a full, thorough, adequate investigation (of the visit) as well as sufficient follow-up".
Suleimani has also reportedly been sighted visiting Iranian-backed forces in Iraq.
Iran is providing military support to Syrian President Bashar al-Assad's regime, which is embroiled in a four-year-old conflict with various rebel groups.
Tehran is also supporting several groups fighting Islamic State jihadists in Iraq and Syria.
Suleimani is considered a key figure in the provision of that military assistance.
Syria has come under renewed diplomatic focus following the July nuclear deal with Iran, which has announced it will present a Syria peace plan to the UN.