BEIRUT (REUTERS) - More than 1,000 soldiers deployed by Iran to Syria to back the government side in its civil war have been killed, an Iranian official said, underlining Teheran's increasing presence on frontlines of the conflict.

It was a major increase in the reported death toll from just four months ago, when the Islamic Republic announced that 400 of its soldiers had died on Syria's battlefields.

Iran has been sending fighters to Syria since the early stages of the more than five-year-old war to support its ally, President Bashar al-Assad, against rebels and Islamist militants, including Islamic State in Iraq and Syria (ISIS), trying to topple him.

Although many of the soldiers the Shi'ite Muslim Iran sends are its own nationals, it is casting its recruitment net wide, training and deploying Shi'ites from neighbouring Afghanistan and Pakistan as well.

Half of the death toll reported in August were Afghan citizens. "Now the number of Iran's martyrs as defenders of (the) shrine has exceeded 1,000," Mr Mohammadali Shahidi Mahallati, head of Iran's Foundation of Martyrs, which offers financial support to the relatives of those killed fighting for Iran, was quoted as saying by Tasnim news agency.

Iran alludes to its fighters in Syria as "defenders of the shrine", a reference to the Sayeda Zeinab mosque near Damascus, which is where a granddaughter of the Prophet Mohammad is said to be buried, as well as other shrines revered by Shi'ites.

Many Iranians initially opposed involvement in Syria's war, harbouring little sympathy for Mr Assad. But now they are warming to the mission, believing that the Sunni Islamist ISIS is a threat to the existence of their country that is best fought outside Iran's borders.

With public opinion swinging increasingly behind the cause, numbers of volunteer fighters have soared far beyond what Teheran is prepared to deploy in Syria.