WASHINGTON — President Donald Trump’s national security adviser is calling on Russia to re-evaluate its support for Syrian President Bashar Assad, leaving open the possibility of additional U.S. military action against Syria.

In his first televised interview, H.R. McMaster pointed to dual U.S. goals of defeating the Islamic State group and removing Assad from power. As Secretary of State Rex Tillerson was making the Trump administration’s first official trip this week to Russia, McMaster said Russia will have to decide whether it wanted to continue backing a “murderous regime.” Trump is weighing next steps after ordering airstrikes last week.

“It’s very difficult to understand how a political solution could result from the continuation of the Assad regime,” McMaster said on “Fox News Sunday.”

“Russia should ask themselves ... Why are we supporting this murderous regime that is committing mass murder of its own population?”

He said Russia should also be asked how it didn’t know that Syria was planning a chemical attack since it had advisers at the Syrian airfield.

“Right now, I think everyone in the world sees Russia as part of the problem,” McMaster said.

After last Tuesday’s chemical attack in Syria, Trump said his attitude toward Assad “has changed very much” and Tillerson said “steps are under way” to organize a coalition to remove him from power.

But as lawmakers called on Trump to consult with Congress, Trump administration officials sent mixed signals on the scope of future U.S. involvement. While Nikki Haley, the U.S. ambassador to the United Nations, described regime change in Syria as a U.S. priority and inevitable, Tillerson suggested that last week’s American airstrikes in retaliation for the chemical attack hadn’t really changed U.S. priorities toward ousting Assad.