Defense Secretary James Mattis said Tuesday the U.S. would stay focused on going after the Islamic State (ISIS) and avoid getting drawn into a war with Syria, despite the White House warning the regime not to conduct another chemical weapons attack on its own people.

“How do you avoid mission creep?” Mattis said. “You stay focused on where the enemy is and you set up any number of coordination efforts if you’re getting near converging forces, either Assad regime or Russian. You have to assume there are either Iranian officered or Lebanese Hezbollah elements with them.”

“So what we do is we keep moving against ISIS,” he told reporters traveling with him to Germany for a NATO meeting with counterparts.

The White House on Monday said it detected potential preparations for another chemical weapons attack in Syria and warned the regime against doing so.

“The United States has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children. The activities are similar to preparations the regime made before its April 4, 2017 chemical weapons attack,” White House Press Secretary Sean Spicer said in a statement.

“As we have previously stated, the United States is in Syria to eliminate the Islamic State of Iraq and Syria. If, however, Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price,” he said.

Pentagon spokesman Navy Capt. Jeff Davis said the White House statement came after aerial surveillance showed activity at Syria’s Shayrat air base, where the Syrian regime launched an April 4 chemical weapons attack from, killing scores of civilians. President Trump ordered a cruise missile attack against the air base in response to the attack.

Both Syria and Russia blasted the most recent White House statement, with a Syrian minister suggesting the U.S. was trying to gain leverage before a new United Nations diplomatic offensive and with Russia calling the threat “unacceptable.”

Chief Pentagon Spokesperson Dana White said the Pentagon would not discuss intelligence matters, but said, “the Assad regime’s use of chemical weapons against its own people is well-documented.”

Mattis acknowledged, however, that the battle space was getting more crowded, with U.S.-backed Syrian forces closing in on ISIS’s stronghold of Raqqa and Russian and Iranian forces backing Syrian regime forces who are purportedly fighting ISIS as well.

The U.S. on June 18 shot down a Syrian regime fighter jet after it dropped bombs near the U.S.-backed forces on the ground.

Iranian-backed forces have also drawn closer to a military base in southern Syria where the U.S. is training more forces in the fight against ISIS.

“However, as you mix more forces more closely together, what worked before for deconfliction won’t. It’s going to take more precision,” Mattis said. “You’ve got to really play this thing very carefully and the closer we get, the more complex it gets.”

The U.S. began its air war in Syria in September 2014, supporting local Syrian forces who agreed to go after ISIS. Russia began its own air war there in September 2015, pledging to go after ISIS but shoring up the regime against rebels in its civil war instead. The two sides soon after established a deconfliction channel to avoid midair collisions or any accidents.

Mattis said with the various forces converging on ISIS in the Euphrates River Valley, it may require dividing it into smaller zones between the U.S., Syria, and Russian forces.

“As long as it is worked out by the commanders and enough people know about it in sufficient time, there are ways that are proven that we can do this,” he said.