US Vice President Joe Biden said on Wednesday that he understood the "intense feeling" in Turkey over the US-based preacher Fethullah Gulen whom Ankara accuses of masterminding the July 15 failed coup and wants to see extradited.

Biden´s one-day trip to Ankara was largely aimed at repairing damage to ties caused by the botched coup and Turkey´s impatience over the failure of the US to rapidly surrender Gulen.

Speaking at a news conference with Turkish Prime Minister Binali Yildirim, Biden said work was continuing by US legal experts to evaluate the evidence produced by Turkey that needs to "be supplied to an American court" for an extradition.

"I understand the intense feeling your government and the people of Turkey have about him (Gulen). We are cooperating with the Turkish authorities."

He added that the US has "no, no, no, no interest whatsoever in protecting anyone who has done harm to an ally. None. But we need to meet the legal standard requirement under our law."

He said it would be an impeachable offence for US President Barack Obama to order the extradition of a foreign national.

"We have no reason other than to cooperate with you (Turkey)... It always takes time... It is never understood why the wheels of justice move deliberately and slowly. It is totally understandable why the people of Turkey are angry," he said.

Turkish officials have warned that if Pennsylvania-based Gulen is not extradited, relations will suffer further and anti-American sentiment will deepen in the country.

Yildirim said if the process of extraditing Gulen is expedited, Turkish people´s disappointment would be dispelled "very quickly".

´I do apologise´

Meeting later with President Recep Tayyip Erdogan, Biden sought to counter head-on Turkish disappointment that Western officials had not come sooner to express solidarity after the coup.

"The American people stand with you," Biden told reporters, sitting next to Erdogan at the presidential palace in Ankara. "But I do apologise. I wish I could have been here earlier."

But he added that Ankara needed to produce more if it was going to ensure the extradition of Gulen.

"How long it will take will depend on how much evidence is able to be presented. Thus far, until yesterday, there has been no evidence presented about the coup.

"When you go into an American court you cannot say ´this is a bad guy´. You have to say ´this is a guy or a woman who committed the following explicit crime´" said Biden.

He also expressed astonishment at speculation inside Turkey that the United States wanted to protect Gulen.

"Can you imagine us being happy with another military state (in Turkey)? We did not get on so well with your previous military states. So what motive could we possibly have?" he added.

For his part, Erdogan said Gulen´s extradition "as soon as possible" was Turkey´s biggest priority