Russian Su-34 warplanes took off from an airbase in Iran on Wednesday and conducted an airstrike against Islamic State in the Syrian province of Deir al-Zour, the Defence Ministry in Moscow said.
However, the speaker of Iran's parliament, Ali Larijani, denied that Russian warplanes have been using Iranian territory to launch such attacks.
"We have made no airbase available to Russia or any other country," Larijani told parliament in comments carried by Iran's IRNA news agency.
However, he added that Russia is an important ally and has had close cooperation with Iran regarding the Syria conflict.
The Russian Defence Ministry said Wednesday's attack hit two Islamic State command points and large training camps, "eliminating more than 150 militants," according Russian state news agency TASS.
On Tuesday, Russian warplanes used an Iranian airbase for the first time for military actions in Syria, TASS reported.
The airbase, in Iran's western Hamadan region, is much closer to Islamic State targets than the Khmeimim airbase, which Russia had previously been using, in coastal northern Syria.