Donald Trump says he is 'very disappointed' with Turkish President Recep Tayyip Erdogan over his refusal to free US pastor Andrew Brunson.

Trump suggested in comments to Bloomberg News that he had expected Brunson to be freed after helping in the release of a Turkish citizen detained in Israel.

'I'm disappointed in him,' Trump told Bloomberg, adding that the issue was 'too dear to my heart.' 

'I got somebody back for him,' he continued. 'I'm very disappointed in him, but we'll see how it all works out.'

Brunson spent over two decades working as a pastor in the southwestern city of Izmir. But his arrest two years ago has triggered a diplomatic crisis and strained relations between the US and Turkey.

The evangelical pastor was recently moved from prison to house arrest as he continues to fight espionage and terrorism-related charges in the courts. His most recent appeal was rejected.

The US has intensified pressure on Turkey to release Brunson, worsening the country's economic crisis by doubling tariffs on steel and aluminium, and placing sanctions on two Turkish ministers.

Last week, the American Center for Law and Justice, which is lobbying for Brunson's release, filed a submission with the UN Human Rights Council urging it to take action.

The submission stated: 'The U.N. has long stood to protect the human rights of all people. In intervening on behalf of Pastor Brunson, the U.N. will once again demonstrate its commitment to ensuring that freedom of religion exists for all people, regardless of their faith.' 

On Monday, Turkey's top appeals court judge appeared to refer to the Brunson case without mentioning the pastor by name.

According to Hurriyet Daily News, Judge İsmail Rüştü Cirit suggested foreign states need to respect Turkey's sovereignty in its rulings on foreign citizens. 

'The only and absolute power that can rule on the arrest of a foreign citizen in İzmir and decisions about his trial are the independent and impartial courts, which use their judicial power on behalf of the Turkish nation,' Cirit said.

'Foreign states that claim to sustain ideals of democracy, human rights and the state of law should first respect the Republic of Turkey's sovereignty, which is protected by international law.'

The next hearing in Brunson's trial is scheduled for October 12.