The fall of the Assad regime in Syria could spark further tensions in the already strained relationship between the U.S. and Turkey, which have repeatedly found themselves at loggerheads in Syria. Lawmakers expressed concerns on Monday about the potential for expanded Turkish attacks on U.S.-backed forces.

Turkey and Turkish-backed forces have repeatedly clashed with and attacked U.S.-backed forces in the country, including Kurdish fighters, whom the Turkish government has described as terrorists. Turkey also backs Hayat Tahrir al-Sham, the rebel group and U.S.-designated terrorist organization that ousted the Assad government. The instability could create an opportunity for Ankara to expand its influence inside Syria.

Sen. Markwayne Mullin (R-OK) said he does not believe the U.S. has a role to play in deciding whether the rebel forces who ousted Assad will remain in power. Still, he acknowledged that it is in the U.S.’ national security interest to prevent Turkey from taking out Kurdish forces, which are holding over ten thousand ISIS fighters in refugee camps across the country. 

“There’s some concerns of that. That’s a fact, the Kurds are holding prisoners. We’re well aware of that. We want them to keep holding them,” Mullin said. “We don’t know how this will be. We’re making assumptions on how this new regime is going to handle it. Right now, they haven’t attacked the Kurds. Right now, they’ve been okay. I think you’ve gotta be more worried about what Turkey does with the Kurds than you do about the new regime in Syria. That’s interesting dynamics.”

Sen. James Lankford (R-OK) said that the U.S. has “got to defend the Kurds.”

“As their allies, we need to stay very engaged,” Lankford said. “We need to send a very clear statement to the Turks that we stand with the Kurds and believe in their opportunity to have freedom.”

Sen. Chris Murphy (D-CT) said that the U.S.’ priority in this transition period needs to be “a very strong message sent to all of our friends and adversaries in the region that there will be a cost if our partners are deliberately targeted in the days and months to come.”

Sen. Richard Blumenthal (D-CT) said he’s “concerned about Turkey trying to exploit the instability in Syria.”

“I think we should attempt to play a constructive role in making sure that there is stable, pluralistic rule there, and that we avoid the kind of bloodbath that followed in other countries where the government fell apart.”

Sen. Kevin Cramer (R-ND) said Turkey’s involvement up to this point is “eerie” and said it “wouldn’t be shocking” if Turkey launched an escalated campaign against the Kurds. 

“With that whole issue, and the solution has been elusive for a long time, about what to do for and with the Kurds. Intervention comes at a very high cost, and I don’t see a lot of appetite in the United States for that type of intervention. It saddens me a lot, actually. Turkey is an outlier. They’re the unknown.”

Sen. Thom Tillis (R-NC) said that Turkey may attempt to fill the vacuum left by Assad and his allies, but expressed some acceptance of such an outcome.

“They are also a NATO ally. So I’d like to think at some level that that could be a positive look,” Tillis said, expressing some level of understanding for Turkey’s concerns about Kurdish forces. “I was in Turkey back when I was in the state legislature, the day that the PKK murdered 22 Turkish service members. I was literally in Ankara having lunch with parliament members at the time.”