Western allies backing opponents of Assad have expressed concern about the role that Hezbollah fighters are playing in the Syrian civil war.
U.S. State Department spokesman Patrick Ventrell said Hezbollah's role in the Syrian war was making matters in the region worse.
"We reject Hezbollah's efforts to escalate violence inside Syria," he said this week.
Hezbollah has claimed military support from Iran. It helped Syrian forces capture territory near the Lebanese border once held by rebel forces.
Iranian Foreign Ministry spokesman Abbas Araqchi said Iran had little direct military influence on the ground in Syria.
"The real enemies of Syria make such claims to provoke that country's people (against Iran) and divert developments (in Syria) in the wrong direction," he was quoted by Iran's state-funded news agency Press TV as saying.
Iran is one of the strongest regional allies of the Assad regime.
U.S. Secretary of State John Kerry said this week that considerations would be made to support Syrian rebels as they "fight for the freedom of their country" should Assad fail to show a commitment to finding a political settlement to the crisis.
Some rebel groups fighting against Assad are allied with al-Qaida.