President Joe Biden will meet Monday with Turkish President Recep Tayyip Erdoğan, the first face-to-face discussion between the two leaders after acknowledging a strained bilateral relationship.

The meeting, which will take place on the sidelines of the NATO leaders summit, will be an opportunity for Biden and Erdoğan to address the thorny issue of U.S. sanctions over a Russian-made missile system.

Under the Countering America’s Adversaries Through Sanctions Act, or CAATSA, any foreign government working with the Russian defense sector finds itself in the crosshairs of U.S. economic sanctions.

In December, the Trump administration slapped CAATSA sanctions on Turkey after the NATO ally purchased a multibillion-dollar acquisition of a Russian missile system. The S-400, a Russian mobile surface-to-air missile system, is said to pose a risk to the NATO alliance as well as the F-35, America’s most expensive weapons platform.

The move further stoked tensions between Washington and Ankara in the weeks ahead of Biden’s ascension to the White House.

National security advisor Jake Sullivan told reporters on Sunday that Biden and Erdoğan will discuss “issues in our bilateral relationship,” without specifically naming the U.S. sanctions.

Sullivan also said that the two are expected to consult on a range of regional security issues, spanning from Syria to Libya to the eastern Mediterranean. He added that Biden will also have the chance to consult with his Turkish counterpart on how to counter China and Russia.