David Jackson, and Gregory Korte

USA TODAY

WASHINGTON — The White House echoed its threat of additional missile strikes against Syria on Monday if Bashar al-Assad's government again uses chemical weapons — and perhaps other types of weapons as well.

"The sight of people being gassed and blown away by barrel bombs ensures that if we see this kind of action again, we hold open the possibility of future action," White House spokesman Sean Spicer said.

Assad's government has frequently used barrel bombs — improvised explosives designed to kill or maim with shrapnel — against civilians.

Secretary of Defense James Mattis said Monday that last week's strike had damaged or destroyed 20% of Syria's operational aircraft, in addition to fuel and ammunition depots and air defense capabilities. And despite reports that Syria was once again using the Shayrat airfield, he said the inability to refuel there made it only of "idle military interest."

"The Syrian government would be ill-advised ever again to use chemical weapons," Mattis said in a statement.

While administration officials appear divided on whether to call for Assad's removal, Spicer said that Trump's goals in Syria are defeating the Islamic State while "creating the political environment necessary for the Syrian people to have a new leadership there ... I can't imagine a stable and peaceful Syria where Bashar al-Assad is in power."

Asked whether the administration believes Assad has committed war crimes, Spicer said "there is a court that decides those things, and obviously there's a reason that."

In interviews over the weekend, Trump administration appeared to send mixed signals about its policy on Syria. U.N. ambassador Nikki Haley said Assad should be removed from power, while Secretary of State Rex Tillerson suggested that decision should be made by the Syrian people and that the emphasis should be defeating the Islamic States, also known as ISIS. Those statements are not "mutually exclusive," Spicer said.

"As you reduce ISIS' strength, as you de-escalate the conflict in Syria, the political environment to remove him becomes stronger and stronger," the White House spokesman said.

But he also said Russia, a major supporter of the Assad regime, needs to be part of the solution. U.S. officials have alleged that Russia knew that Syria had retained chemical weapons, even after a Russian-brokered agreement to eliminate them in 2014.

Tillerson headed to Moscow on Monday for talks with his Russian counterpart. "I think with respect to Syria in particular, I think we need to remind them of the commitments that they've made and the commitments that Syria's made," Spicer said.

Read more:

The first 100 days of the Trump presidency

U.S. will take additional action 'as necessary' on Syria, Trump tells Congress

Trump officials send mixed signals on what's next for foreign policy