The US Defense Secretary Mark Esper said Monday that he wants an explanation from Ankara over threats to close two strategic military bases used by the United States in Turkey.

On Sunday, Turkey’s President Recep Tayyip Erdogan threatened to close the Incirlik and Kurecik bases while speaking to a pro-government television channel.

Earlier this week, Turkish Foreign Minister Mevlut Cavusoglu also brought up the issue of closing the two bases to the US if Washington imposes sanctions.

Mark Esper said that he would need to speak to his counterpart, defense minister Hulusi Akar, “to understand what they really mean and how serious they are.”

“If the Turks are serious about this, I mean, they are a sovereign nation, to begin with… they have the inherent right to house or not to house NATO bases or foreign troops,” Mark Esper said on a plane as he flew back from Belgium, where he had attended the 75th anniversary of the Battle of the Bulge.

“I think this becomes an Alliance’s matter, their commitment to the Alliance, if indeed they are serious about what they are saying,” he added.

In October, the US Vice President Mike Pence and Turkish officials had announced after having a high-stakes talk that Turkey agreed to suspend its Syria offensive for five days and will end the assault if Kurdish-led forces withdraw from a safe zone along the border.

US President Donald Trump had sent a letter to his Turkish counterpart Erdogan, after which Trump warned Erdogan on Syria offensive and called him “Don’t be a fool”.

Trump had announced that he will authorise sanctions against Turkish officials, raise steel tariffs and end negotiations on a USD 100 billion trade deal.

On pulling back of US troops, Trump said that only a small contingent will remain at the Tanf Garrison in southern Syria to disrupt remnants of the ISIS.

During the 2016 presidential campaign, Trump had promised to bring back US troops from Syria and Afghanistan.

Turkey faces US sanctions over its decision to buy the Russian S-400 missile defense system, despite warnings from Washington.

Relations between the two NATO allies have also been strained over Turkey’s military incursion in northeast Syria against Kurdish forces that partnered with the US against IS. After the US military withdrew from the Turkish-Syrian border, Turkey struck a deal with Russian President Vladimir Putin to patrol northern Syria.

(With inputs from agency)