The Syrian government is sending members of its irregular militias for guerrilla combat training at a secret base in Iran, in a move to bolster its armed forces drained by two years of fighting and defections, fighters and activists said, according to Reuters.

The discreet program has been described as an open secret in some areas loyal to President Bashar al-Assad, who is trying to crush a revolt against his family's four-decade hold on power.

Reuters interviewed four fighters who said they were taken on the combat course in Iran, as well as opposition sources who said they had also been documenting such cases.

Israel's intelligence chief and a Western diplomat have said Iran, Assad's main backer, is helping to train at least 50,000 militiamen and aims to increase the force to 100,000 - though they did not say where the training occurred.

No one at Iran's foreign ministry was available for comment, but Iranian officials have repeatedly denied military involvement in the Syrian conflict, saying they have only provided humanitarian aid and political support for Assad.

A Syrian government security source, who declined to be named, denied that Syria was sending fighters to Iran.

But if the reports by Syrian fighters are true, the move to train combatants in Iran suggests that their country's increasingly regionalized conflict has grown well beyond - and could even outlast - a battle for power between Assad's circle and the opposition.

The fighters also appear to come largely from minority groups that have supported Assad against the mostly Sunni Muslim-led uprising. Such a move could exacerbate the dangerous sectarian dimensions of a conflict that has turned into a civil war that has cost the lives of more than 70,000 people, Reuters reported.