US President Donald Trump has threatened Turkey with 'large sanctions' if it does not release imprisoned pastor Andrew Brunson.

Brunson, 50, who has lived in Turkey for 23 years and led a church in Izmir, has been detained for 21 months on charges of espionage and terrorism, all of which he denies.

He has this week been moved from prison to house arrest on health grounds and is due to face another hearing in October. If he is found guilty he could face a sentence of up to 35 years.

Trump tweeted yesterday from aboard Air Force One: 'The United States will impose large sanctions on Turkey for their long time detainment of Pastor Andrew Brunson, a great Christian, family man and wonderful human being. He is suffering greatly. This innocent man of faith should be released immediately!'

Trump's tweet followed a similar threat by Vice President Mike Pence, who said at a religious freedom conference hours earlier: 'To President [Recep Tayyip] Erdogan and the Turkish government, I have a message on behalf of the president of the United States of America: release Pastor Andrew Brunson now or be prepared to face the consequences.'

A spokesman for Turkish President Tayyip Erdogan called Washington's threats unacceptable and damaging to the US-Turkey alliance.

'The United States must reconsider its approach and adopt a constructive position before inflicting further damage to its own interests and its alliance with Turkey,' Erdogan's spokesman, Ibrahim Kalin, said in a written statement.

Turkish Foreign Minister Mevlut Cavusoglu called US Secretary of State Mike Pompeo to discuss the threat of sanctions, Turkish and State Department officials said, without elaborating.

Trump's threat comes as the Turkish lira has lost one-fifth of its value this year over concerns about the central bank's ability to rein in double-digit inflation.

'Turkey's economy is very shaky,' said Anders Aslund, a senior fellow at the Atlantic Council in Washington, told Reuters. 'Erdogan has taken command over the economy and he has negative macroeconomic credibility, so we expect a serious current account crisis. US threats of sanctions aggravate it.'

As a sign of how seriously the US is taking the Brunson case, it is re-evaluating all military sales to Turkey including Lockheed Martin F-35 fighters and the S-400 missile system which can be used to shoot them down, according Aviation Week's Lee Hudson. A rift in military co-operation between Turkey and the US could have implications for NATO, as Turkey is an important ally.