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Michael Morell made the comments during an interview with American broadcaster CBS.

During Monday's interview, Morell said that Washington should take revenge for US troops killed by Shia militias following the Iraq invasion and occupation from 2003.

Morell also said that Washington should play a more "aggressive" role in the Syrian war.

"When we were in Iraq, the Iranians were giving weapons to the Shia militia, who were killing American soldiers," Morell CBS.

"The Iranians were making us pay a price. We need to make the Iranians pay a price in Syria. We need to make the Russians pay a price."

When questioned, he explained that "paying the price" meant killing Russian and Iranian troops and militants fighting alongside Syrian regime forces.

"I want to go after those things that Assad sees as his personal powerbase. I want to scare Assad."

Morell is a 33-year veteran of the CIA and twice served as acting head of the spy agency.

This week, the former intelligence chief accused presidential hopeful Donald Trump of being an "unwitting agent" of Russia.

	Russian military intervention has turned the tide of the war [Getty]

Morell has the Democrat's presidential candidate Hillary Clinton who has supported military action in Syria to oust Assad.

"The specific criticisms directed at Clinton from those who find her too hawkish are well-known," Michael Brenner, professor of international affairs at the University of Pittsburgh, wrote in °®Âþµº on Friday.

"They include her vote in favour of the Iraq war; her cheer-leading for the global war on terrorism in all its aspects; her collaboration with the Gates-led faction to push Obama into a major Afghan escalation; her advocacy of direct military action in Syria to unseat Assad."

The Obama administration has long insisted that only Assad - and his Russian and Iranian sponsors - can end the five-year war.

Washington's efforts to get Russia to force Assad to step down have run into the sand with Russia remaining either unwilling or unable to help.

Russian aerial assaults on moderate rebel groups have met no response from the US, forcing allies to seek the protection of larger, better-armed militant groups, including those linked to al-Qaeda.

Russian and Iranian troops - along with Iraqi Shia militias and Lebanese outfit Hizballah - are thought to be the military lynchpin behind Assad's continued rule over large parts of Syria.

Activists and NGOs say that Russian air raids have killed thousands of civilians, and deliberately targeted hospitals and markets.

Meanwhile, hundreds - possibly thousands - of Iranian Revolutionary Guard units and Tehran-sponsored militants have fought on the ground, and become the main thrust behind the regime's military offensives.

Rebels criticise the US for not backing their movement to overthrow Assad, and covertly working with Russia to target rebel groups viewed as "Islamist".

US have been training and arming Syrian Arab and Kurdish forces fighting the Islamic State group, but it is not known if they are actively supporting rebel outfits fighting the regime despite persistent rumours.