By Parisa Hafezi

ANKARA, May 18 (Reuters) - Iran's elite Revolutionary Guards (IRGC) said on Wednesday many Iranians have volunteered to fight in Syria in support of President Bashar al-Assad's war against "terrorism", the semi-official Tasnim news agency reported.

Shi'ite-dominated Iran is Assad's main regional ally and has provided military and economic support for his conflict with range of Sunni Muslim rebel and militant groups including Islamic State and the al Qaeda-affiliated Nusra Front.

"Many young Iranians from different parts of the country and of different ethnicities have volunteered to go to Syria ... to help the Syrian government and the Syrian people in their fight against terrorism...," Tasnim quoted the head of the IRGC's public relations office, General Ramezan Sharif, as saying.

Sharif gave no figures for the number of volunteers.

Assad's government has branded all those seeking his removal - whether Islamist or relatively secular insurgents, as well as the pro-democracy demonstrators whose peaceful uprising launched Syria's conflict in 2011 - as "terrorists".

The Islamic Republic denies having any conventional armed forces in Syria, but has acknowledged having military advisers and volunteers from the IRGC there to help Assad's forces.

Last month, however, Tehran said military commandos had also been dispatched to Syria as advisers, suggesting it was using its regular army as well as forces from the IRGC.

However, Iran's army chief later said those personnel were volunteers working under IRGC supervision, and that the regular army was not directly involved.

Iran has two distinct armed forces - a regular army serving as a national defence force, and the IRGC that was created after the 1979 Islamic Revolution to protect the Islamic Republic against both internal and external adversaries.

In the past few months Iran has sustained serious losses in Syria, including several high-ranking members of the IRGC. Iranian media have reported the death of over 100 members of the IRGC and its affiliated volunteer, the Basij militia, in Syria.

The United States, Saudi Arabia and some other Western and Gulf states plus Turkey back rebels fighting to remove Assad, who has military support from Russia and Iran.

"Syria is the golden ring of resistance front," Ali Akbar Velayati, top adviser to Iranian Supreme Leader Ayatollah Ali Khamenei, told Iran's Student News Agency ISNA.

"If Assad and the Syrian people fail in their fight against Takfiri (hardline Sunni Islamist) groups..., their next target will be (Shi'ite-led) Iraq, followed by Iran," he said.

(Editing by Mark Heinrich) ((parisa.hafezi@thomsonreuters.com; +90 532 176 3452; Reuters Messaging: parisa.hafezi@thomsonreuters.com))