US Ambassador to the United Nations Nikki Haley and Secretary of State Rex Tillerson today appeared to offer differing views on the future of Syrian President Bashar al-Assad.

Their comments highlight the degree to which questions remain about the nature of US policy in Syria after President Donald Trump authorised missile strikes against a government air base believed to have been involved in the deployment of chemical weapons against civilians last week.

Haley indicated in an interview on CNN that the United States does not see a peaceful political resolution for Syria's civil war as long as Assad remains in power.

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


But speaking on ABC, Tillerson emphasised that the Syrian people would eventually decide Assad's fate.

Instead, Tillerson emphasised the Administration's priority in defeating Isis (Islamic State) forces in Syria and said little about the US preference on Assad's future.

"Our priority is first the defeat of Isis," Tillerson said. "Once we can eliminate the battle against Isis, conclude that and it is going quite well, then we hope to turn our attention to ceasefire agreements between the regime and opposition forces.

"In that regard, we are hopeful that we can work with Russia and use their influence to achieve areas of stabilisation throughout Syria and create the conditions for a political process through Geneva in which we can engage all of the parties on the way forward and it is through that political process that we believe the Syrian people will lawfully be able to decide the fate of Bashar al-Assad," he added.

The apparent dissonance did not go unnoticed. Speaking later on ABC News, Senator Marco Rubio criticised Tillerson for suggesting that the objective of defeating Isis can be achieved as long as Assad remains in power.

"This idea that we're going to get rid of Isis and then we'll hopefully use Assad and others to come up with a solution. It's not going to work," Rubio said. "There seems to be a difference between what Ambassador Haley is saying, and what she said last night that Assad really has no future, and what I heard this morning from Secretary Tillerson.

"You cannot have a stable Syria without jihadist elements on the ground with Bashar al-Assad in power," he added.

In an appearance on Fox News - his first television interview since taking office as National Security Adviser - General H.R. McMaster was pressed to reconcile the seeming contradictions between the positions advanced by Tillerson and Haley.

"What Ambassador Haley pointed out was, it's very difficult to figure out how a political solution could result from the continuation of the Assad regime," McMaster said.

"We're not saying that we are the ones who are going to effect that change. What we're saying is, other countries have to ask themselves some hard questions. Russia should ask themselves ... why are we supporting this murderous regime that is committing mass murder of its own population and using the most heinous weapons available.

"While people are really anxious to find inconsistencies in those statements, they are in fact very consistent in terms of what is the ultimate political objective in Syria," he added.

Pressed by Fox host Chris Wallace pointed explain Tillerson's statements that destroying Isis is the US priority, McMaster responded: "That's exactly what we're saying".

He added: "There has to be a degree of simultaneous activity, as well as sequencing the defeat of Isis first. What you have in Syria is a very destructive cycle of violence perpetuated by Isis, obviously, but also by this regime and their Iranian and Russian sponsors.

"And the resolution of the conflict will entail both of the elements that you're talking about," he continued.