President Recep Tayyip Erdoğan discussed relations between Turkey and Ukraine, as well as regional developments, with Ukrainian counterpart Volodymyr Zelenskyy, a statement by the Presidential Communications Directorate said Tuesday.

According to the statement, the two leaders talked about means to further enhance bilateral ties in a phone call.

Zelenskyy said bilateral ties had made their armies stronger.

Ukraine has bought and deployed Turkish drones in the war against Russian-backed forces in its eastern Donbass region, angering Russia. Russia's subsequent troop movements on Ukraine's borders have sparked concern in the West.

Zelenskyy in a tweet said the partnership with Turkey was deepening. "It's already strengthening the armed forces of our countries. The expected FTA will also contribute to economic growth."

The phone call came amid the United States' warning that Russia may be planning to invade Ukraine as Moscow amasses forces on its western border.

Recently, U.S. officials, including Secretary of State Antony Blinken and the Pentagon, have said they are closely monitoring the military buildup, but Bloomberg reported that the Biden administration has gone a step further and officially warned European Union allies of a potential Russian military offensive.

Shortly after the warning, NATO Secretary-General Jens Stoltenberg warned Russia on Monday that the western military alliance stands by Ukraine amid the unusual deployment of Russian troops on Ukraine's borders.

Turkey, a NATO member, has criticized Moscow's annexation of Crimea and voiced support for Ukraine's territorial integrity. The United States and United Nations General Assembly view the annexation as illegal as well.

Fighting between Ukrainian government forces and pro-Russian separatists in Donbass has seen more than 13,000 people killed since 2014, according to the U.N.

The region is one of several sources of friction between Russia and Ukraine.