Iran's defence minister insisted on Saturday that the country has never sent "military forces" to Syria, reiterating its denial of allegations by Western powers backing Syrian rebels.

"The Islamic Republic of Iran has never sent military forces to Syria and will never do so," Ahmad Vahidi said in a statement reported by Iranian media.

"Iran does not believe in sending military troops to Syria based on its policies towards the resolution of the conflict," he said, referring to Tehran's calls for a political solution to the war estimated to have cost more than 94,000 lives since 2011.

He did not specify whether he was referring to combat troops or military advisors.

In September, the chief of Iran's elite Revolutionary Guards, Mohammad Ali Jafari, said that members of his foreign operations Quds Force unit were in Syria but only to provide its government with "counsel and advice."

Iran is accused by Western and Arab countries which back rebels fighting its ally President Bashar al-Assad of supplying weapons and sending military forces to the Syrian military.

A top US official said on Tuesday that Iranians were working alongside their Shiite Lebanese ally Hezbollah fighters to back Syrian troops battling to retake the rebel stronghold of Qusayr, near the Lebanese border.

"It is the most visible effort we have seen by Hezbollah to engage directly in the fighting in Syria as a foreign force, and we understand there are also Iranians up there," said the official, speaking on condition of anonymity.

Such accusations are also raised by the Syrian armed opposition which accuses Iran of "killing Syrians."

"Thousands of Iranian forces and its terrorist collaborators from Hezbollah are invading Syria. The invading fighters are besieging, shelling and trying to assault several towns, among them Qusayr and Homs ...," acting National Coalition chief George Sabra told AFP in Istanbul on Saturday.

Iran, opposed to Assad's removal from power, regards many Syrian opposition groups as "terrorists" but has repeatedly rejected the allegations of playing a military role on the ground.

A top Iranian military commander said in early May that Iran was ready to "train" the Syrian army if Damascus sought such assistance.