A political solution to the war in Syria is not possible with President Bashar al-Assad in power, the US ambassador to the UN has said in an apparent hardening of the Trump administration's position.

Defeating the Islamic State of Iraq and the Levant (ISIL) group, pushing Iranian influence out of Syria, and the removal of Assad are priorities for the US administration, Nikki Haley said in an interview to CNN, which will air in full on Sunday.

"We don't see a peaceful Syria with Assad in there," she said.

The comments represented a departure from what Haley had said before the US hit a Syrian airbase with 59 Tomahawk missiles on Thursday in retaliation for what it said was a chemical weapons attack by Assad's forces on Syrian civilians.

President Donald Trump ordered the missile strike after watching television images of infants suffering from chemical weapons injuries.

"You pick and choose your battles and when we're looking at this, it's about changing up priorities, and our priority is no longer to sit there and focus on getting Assad out," Haley had said on March 30, just days before dozens of Syrian civilians died from chemical weapons injuries.

Rex Tillerson, US secretary of state, seemed to take a more patient stance with regard to Assad, saying on Saturday that the administration's first priority is the defeat of ISIL, also known as ISIS.

Once the threat from ISIL has been reduced or eliminated, "I think we can turn our attention directly to stabilising the situation in Syria," Tillerson said in excerpts from an interview on CBS's Face the Nation.

He said the US is hopeful it can help to bring parties together to begin the process of hammering out a political solution.

On the ground in Syria, meanwhile, fresh air strikes have targeted the same town that was hit by the recent chemical attack, killing at least 17 people, according to the Syrian Observatory for Human Rights (SOHR).

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It is not immediately clear who was responsible for the strikes on Friday and Saturday in Khan Sheikhoun, but only Russian and Syrian aircraft have been bombing the rebel-held areas of Idlib province.

Elsewhere in Syria, at least seven people were killed in air strikes in Eastern Ghouta, near the capital Damascus.

Activists say chlorine gas was dropped, choking at least two people. This was also the site of chemical attacks believed to have been carried out by the Syrian government in 2013, which killed hundreds.

More than 320,000 people have been killed in Syria since the conflict erupted in March 2011 with anti-government demonstrations.