US Defense Secretary Mark Esper has called on Ankara to provide an explanation about threats to close two strategic military bases the United States is using in Turkey.

On Sunday, Turkish President Recep Tayyip Erdogan threatened to close the Incirlik and Korijic bases in response to threats of US sanctions. The bases are located on the southwestern coast of Turkey, near the border with Syria.

The US Air Force uses the Incirlik base to combat the Islamic State in Syria, while the Korijic base includes a major radar station for NATO.

The US Secretary of Defense told reporters that he would need to speak to his Turkish counterpart, Khulusi Akar, "to understand what they really mean and how serious they are."

"They are a sovereign state ... They have the natural right to receive or not receive bases for NATO or foreign forces," Esber added. He continued, "I think it becomes a matter of NATO, their commitment to NATO, if they are really serious about what they say."

Esber also expressed his disappointment with the direction that he said that Ankara was taking "in its distance from NATO and its proximity to Russia."

The United States imposed sanctions on Turkey for buying Russian S-400 missiles, despite warnings from Washington about the consequences of this.

In recent months, relations between the two NATO countries have deteriorated, against the backdrop of Ankara launching an attack on Kurdish YPG in northern Syria.

And last Wednesday, the US Senate Foreign Affairs Committee adopted a proposal to impose harsh penalties on Turkey and its officials against the backdrop of the S400 missile purchase and attack on northern Syria.

Last week, the US Senate unanimously endorsed a resolution that considered genocide to kill Armenians a century ago, in a move that angered Turkey and dealt a blow to the already strained relations between the two countries.