Turkish presidential spokesman Ibrahim Kalin and U.S. National Security Advisor John Bolton discussed the Russian S-400 air defense system via telephone Tuesday, according to presidential sources.

Tensions between the U.S. and Turkey have increased recently with Turkey set to begin receiving the surface-to-air missile system that Washington contends will jeopardize Ankara’s role in the F-35 fighter jet program and could trigger American congressional sanctions.

Turkey decided in 2017 to purchase the S-400 system following protracted efforts to purchase air defense systems from the U.S. with no success.

Kalin and Bolton also discussed bilateral relations, a date for U.S. President Donald Trump's visit to Turkey, and recent developments in Libya.

In early April, Khalifa Haftar, who commands forces loyal to Libya’s eastern government, launched a campaign to capture Tripoli from Libya’s UN-recognized Government of National Accord (GNA).

Libya has remained beset by turmoil since long-serving leader Muammar Gaddafi was ousted and killed in a bloody NATO-backed uprising in 2011.

Since then, the country has seen the emergence of two rival seats of power: one in eastern Libya, to which Haftar is affiliated, and the other, the GNA, in Tripoli.