John Bolton
John BoltonReuters

U.S. National Security Adviser John Bolton met on Monday with the Turkish Ambassador to the United States, Serdar Kilic.

White House press secretary Sarah Sanders said the two “discussed Turkey’s continued detention of Pastor Andrew Brunson and the state of the U.S.-Turkey relationship.”

The case of Brunson, who has been detained in Turkey for more than a year on terrorism charges, is one of the factors that have led to a recent deterioration in relations between Washington and Ankara.

President Donald Trump last week imposed higher tariffs on imports from Turkey, including a 20 percent duty on aluminum and a 50 percent duty on steel.

The Turkish Foreign Ministry has vowed to "retaliate" against the U.S. sanctions, calling the American move a "hostile stance".

Brunson is accused of working with Fethullah Gulen, an Islamic cleric who Turkish President Recep Tayyip Erdogan claims orchestrated a failed coup in 2016.

In addition, a dispute between the countries began last October, when the U.S. mission in Turkey reduced visa services in response to a U.S. mission employee being detained in Turkey.

The Turkish mission in Washington subsequently announced a similar move, with both sides saying they needed to reassess each other's commitment to the security of their personnel.

In December, the U.S. and Turkey resumed full visa services for the other country.