America's war on Islamic militants is not aimed at helping Syrian President Bashar al-Assad stay in power, U.S. Secretary of State John Kerry insisted Friday.

The top U.S. diplomat sought to dispel wariness that the "laser focus" on fighting Islamic State (IS) militants has distracted from efforts to end the three-year civil war in Syria in which over 200,000 people have been killed.

Damascus has long insisted that defeating the "terrorists" must take precedence over everything else, including negotiations about political reform.

And the Syrian regime welcomed the U.S.-led air strikes on its territory as part of that fight against "terrorists" which it insists includes the moderate rebels.

"This campaign is not about helping President Bashar Assad of Syria," Kerry shot back in an opinion piece Friday in the Boston Globe.

"We are not on the same side as Assad -- in fact, he is a magnet that has drawn foreign fighters from dozens of countries to Syria."

"Assad lost legitimacy a long time ago," Kerry stressed.

U.S. efforts to train and equip vetted members of the Syrian opposition to fight IS militants "will promote conditions that can lead to a negotiated settlement that ends this conflict."

But new Iraqi Prime Minister Haider al-Abadi told U.S. reporters on Thursday that he had only backed the air strikes in Syria after being told that Syrian regime officials and the military would not be targeted.

"We had a lengthy discussion with our American friends, and what they emphasized is their aim in Syria is not to destabilize Syria," Abadi said, quoted by the New York Times.

Instead the aim was "to diminish the capability of Daesh," Abadi said, using the Arabic term for the Islamic State group.

The Iraqi prime minister also said the United States had asked him to send a message via one of his advisers to the Assad government that it would not be a target of the strikes.

U.S. officials have said more than 60 countries are now taking part in the anti-IS coalition, "responding with a unity that shows these criminals that we will not allow them to divide us or force their nihilistic vision on helpless people," Kerry wrote.

He pointed to Wednesday's unanimous and binding resolution demanding that countries take action to stem the flow of foreign jihadists to Iraq and Syria.

Even Russia, a staunch Assad ally long accused of funneling funds and weapons to the regime, did not wield its veto during the Security Council.

Russian Foreign Minister Sergei Lavrov said Friday that any action against IS militants in Syria should be in line with international law and "it's very important that such cooperation with Syrian authorities is established."

The fight against IS militants in Syria was raised in "in-depth" talks this week between Kerry and Lavrov, a State Department official confirmed.

"The U.S. and Russia share an interest in defeating the kind of violent extremism that ISIL represents," the senior U.S. official told a small group of reporters.