Iran accused the United States on Saturday of seeking to "topple the legitimate regime" of Bashar al-Assad in Syria under the pretext of fighting against Islamic State (IS) militants.

The secretary of Iran's Supreme National Security Council Ali Shamkhani said Washington was "trying to divert the eyes of the world from its pivotal role in supporting and equipping the terrorists in Syria in a bid to topple the legitimate regime."

And the speaker of parliament, Ali Larijani, warned the United States against launching strikes against Iran's ally Syria.

"The United States is playing with fire in the region and must know that it cannot attack Syria on the pretext of fighting against the Islamic State," he said.

Shamkhani and Larijani were the latest Iranian officials to criticise US efforts to wage war against IS militants.

On Thursday, foreign ministry spokeswoman Marzieh Afhkham cast doubt on the "sincerity" of the coalition Washington is seeking to build.

Last week, Foreign Minister Mohammad Javad Zarif accused the United States of not taking the threat from IS militants in Iraq and Syria seriously.

Iran has been Assad's main regional ally since the uprising against his autocratic rule erupted in March 2011.

Syria and its ally Russia have warned that any unilateral US military action on Syrian territory would be a violation of its sovereignty.

"On the pretext of fighting terrorism, the United States want to pursue their unilateral policies and violate the sovereignty of states," Shamkhani said, quoted by the official IRNA news agency.

The United States is building an international coalition against IS and has said it plans air strikes against the militants on Syrian territory without the consent of Damascus.

Washington reiterated Friday it will not coordinate with Assad.

"The answer to the security challenges is not the Assad regime," State Department deputy spokeswoman Marie Harf told reporters.

"They have created this security vacuum. We are not going to be working with them in this fight," she added.

An Assad advisor, Buthaina Shaaban, was quoted by state news agency SANA earlier Friday saying Damascus must be included as a "key part" of the multinational anti-IS coalition.

But Harf noted that Assad's forces "continue to kill their own people."

Opponents of Assad took up arms in response to a bloody crackdown on peaceful pro-democracy protests inspired by the Arab Spring uprisings.

The three and a half year Syrian civil war has claimed over 190,000 lives and displaced roughly half of the country population, according to the UN.