U.S. President Donald Trump walks across the South Lawn after arriving in Marine One from a recent trip to New York at the White House in Washington, U.S., August 13, 2018. REUTERS/Leah Millis

WASHINGTON (Reuters) - President Donald Trump is frustrated that Turkey has not released U.S. pastor Andrew Brunson, according to the White House, which is ratcheting up pressure on Ankara to free him after two years of detention.

“The president has a great deal of frustration on the fact that Pastor Brunson has not been released as well as the fact that other U.S. citizens and employees of diplomatic facilities have not been released,” White House Press Secretary Sarah Sanders said at a briefing.