Turkey's President Recep Tayyip Erdogan (L) and Russia's President Vladimir Putin shake hands at a joint news conference on Syria following their meeting in Sochi, Russia.

WASHINGTON — President Donald Trump's Wednesday meeting with Turkish President Recep Tayyip Erdogan comes as the two NATO allies navigate several thorny national security issues.

Yet while Erdogan's visit comes on the heels of his military's action against the Kurds in northern Syria, following Trump's decision to withdraw U.S. forces from the area, another U.S.-Turkey dustup has flown under the radar. And it involves an arms deal with Russia.

The U.S. could sanction Turkey for buying Russia's S-400, a mobile surface-to-air missile system that is said to pose a risk to the NATO alliance as well as to America's most expensive weapons system: Lockheed Martin's F-35 jet.

But Turkey shrugged off the potential sanctions, and Trump himself has been sympathetic to Erdogan's decision. In July, Trump said he could not blame Turkey for buying the S-400, a move that resulted in dropping the NATO ally's participation in the F-35 program.

"We'll be talking about the S-400 and we will be talking about the F-35," Trump said Wednesday alongside Erdogan in the Oval Office. "We may report back to you later about that because we are having a second meeting in a little while," he added when pressed on the issue.

Earlier this year, Trump downplayed Turkey's acceptance of the S-400 and placed blame on the Obama administration.

"I don't blame Turkey because there are a lot of circumstances and a lot of ... problems that occurred during the Obama administration," Trump said in July. "This dates back to the Obama administration, which was a disaster."

Read more: Turkey proceeds with deal for Russian missile system despite US and NATO warnings

Trump then did not elaborate on whether he would impose sanctions on Turkey for doing business with the Kremlin. Under the Countering America's Adversaries Through Sanctions Act, which the president signed in August 2017, Turkey could face economic sanctions for buying the Russian-made missile system.

"It's a tough situation. They're getting the S-400 and our statues and everything else — as you do that, you just can't order this equipment," Trump said. "And generally speaking, you can't order equipment period."