U.S. President Donald Trump's national security adviser on Sunday left open the possibility of additional U.S. military action against Syria following last week's missile strike, but indicated that the United States was not seeking to act unilaterally to oust Syrian President Bashar al-Assad.

In his first televised interview, H.R. McMaster pointed to dual U.S. goals of defeating the Islamic State group and removing Assad. But he suggested that Trump was seeking a global political response for regime change from U.S. allies as well as Russia, which he said needed to reevaluate its support of Syria.

"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. "Now, we are not saying that we are the ones who are going to effect that change. What we are 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?"

After last Tuesday's chemical attack in Syria, Trump said his attitude toward Assad "has changed very much" and Secretary of State Rex Tillerson said "steps are underway" to organize a coalition to remove him from power.

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

Pressed to clarify, McMaster said the goals of fighting ISIS and ousting Syria's president were somewhat "simultaneous" and that the objective of the missile strike was to send a "strong political message to Assad." He did not rule out additional strikes if Assad continued to engage in atrocities against rebel forces with either chemical or conventional weapons.

"We are prepared to do more," he said. "The president will make whatever decision he thinks is in the best interest of the American people."

Assad a barrier to political solutions

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United States Ambassador to the United Nations Nikki Haley on Friday questioned how much Russia knew about chemical weapons in Syria during an emergency meeting of the Security Council. (Stephanie Keith/Reuters)

UN ambassador Haley said Sunday no political solutions are possible in Syria if Assad remains in power.

"And a political solution is going to have to happen, but we know that there's not any sort of option where a political solution is going to happen with Assad at the head of the regime," Nikki Haley told CNN's State of the Union program, in an interview to be broadcast later Sunday.

"If you look at his actions, if you look at the situation, it's going to be hard to see a government that's peaceful and stable with Assad," she said.

Haley's comments come two days after she addressed the emergency meeting at the UN Security Council when she warned that the United States is prepared to take further action in Syria if required.

During the meeting, Haley openly questioned Russia's role in Syria and said Russia was supposed to be responsible for removing chemical weapons from Syria.

"It could be that Russia is knowingly allowing chemical weapons to remain in Syria. It could be that Russia has been incompetent in its efforts to remove the chemical weapons. Or, it could be that the Assad regime is playing the Russians for fools, telling them that there are no chemical weapons, all the while stockpiling them on their bases," Haley said.

Meanwhile, Iraq's influential Shia cleric Moqtada al-Sadr has called on the Syrian president to "take a historic heroic decision" and step down, to spare his country further bloodshed.

Sadr, who commands a large following among the urban poor of Baghdad and the southern cities, is the first Iraqi Shia political leader to urge Assad to step down. But his call was wrapped in kind words about the Syrian president and condemnation of the U.S. strikes carried out on a Syrian airbase on Friday, in retaliation for a chemical attack on civilians in a rebel-held area of Syria.

US Syria

This satellite image taken on April 7 shows the northwest side of the Shayrat air base in Syria, shortly after the U.S. Tomahawk missile strikes, carried out in retaliation for a chemical weapons attack against civilians in Syria's Idlib province on April 4. (DigitalGlobe via AP)

Sadr said the U.S. strikes would "drag the region to war" and could help "the expansion of Daesh," the militant group Islamic State in Iraq and Syria (ISIS), which controls parts of Iraq and Syria.

Iraq's Shia-led governments have maintained good relations with the Syrian government throughout the six-year Syrian civil war. Sadr is the only Iraqi Shia leader to keep some distance from Iran, a main backer of Assad along with Russia.

"I think it would be fair for President Bashar al-Assad to offer his resignation and step down in love for Syria, to spare it the woes of war and terrorism ...and take a historic, heroic decision before it is too late," Sadr said in a statement.

U.K. blames Russia for deaths

Michael Fallon, the U.K, defence secretary, wrote in the Sunday Times that Moscow has influence in Syria and surrounding region, and therefore has a responsibility to "pull levers and stop the civil war."

Russia, he said, is responsible "by proxy" for the deaths of civilians in last Tuesday's chemical attack that killed 87 people in Idlib province.

Iranian President Hassan Rouhani, meanwhile, affirmed his support for the Syrian government on Sunday.

In a phone call with Assad, Rouhani called the U.S. missile strike on a Syrian air base a "blatant violation" of Syrian sovereignty, Syrian state media reported. Assad accused the U.S. of trying to boost the morale of "terror groups" in Syria. The government refers to all those fighting against it as terrorists.

Iran has provided crucial military and economic assistance to Assad throughout the conflict. It has organized several Shia militias from around the Middle East to fight in support of Assad's government and has sent troops and officers from its own Revolutionary Guards.

With files from Reuters