FILE PHOTO: U.S. pastor Andrew Brunson reacts as he arrives at his home after being released from the prison in Izmir, Turkey July 25, 2018. Demiroren News Agency/DHA via REUTERS/File Photo

ANKARA (Reuters) - The lawyer of an American Christian pastor on trial on terrorism charges in Turkey said he will appeal to the constitutional court on Wednesday to seek his client’s release.

The case of Andrew Brunson, whose next regular court hearing is on Oct. 12, has become the most divisive issue in a worsening diplomatic row between Ankara and Washington that has triggered a punishing regime of U.S. sanctions and tariffs against Turkey.

“We will appeal tomorrow to the Constitutional Court to lift the house arrest,” lawyer Ismail Cem Halavurt told Reuters on Tuesday.

The evangelical pastor is charged with links to Kurdish militants and supporters of Fethullah Gulen, the cleric blamed by Turkey for a failed coup attempt in 2016. He has denied the accusation - as has Gulen - and Washington has demanded his immediate release.

On Monday, President Tayyip Erdogan said Turkey was determined to fight, within legal and diplomatic frameworks, “this crooked understanding, which imposes sanctions using the excuse of a pastor who is tried due to his dark links with terror organizations.”

Halavurt said he did not expect the constitutional court to make a ruling before the Oct. 12 hearing, “but we want to have completed our appeal before (then)”.

Brunson, who has lived in Turkey for two decades, has been detained for 21 months on terrorism charges and is currently under house arrest.

Donald Trump, who counts evangelical Christians among his core supporters, has become a vocal champion of the pastor’s case.

The U.S. president believed he and Erdogan had agreed a deal to release him in July as part of a wider agreement, but Ankara has denied this.