The head of a top European rights watchdog Wednesday backed a "cleaning up" of Turkish institutions after a failed coup blamed on supporters of US-based preacher Fethullah Gulen.

Despite growing concern over the post-coup crackdown, Council of Europe chief Thorbjorn Jagland said there had been insufficient understanding in Europe about the challenges faced by Turkey.

His comments came as President Recep Tayyip Erdogan, in a rare apology, asked forgiveness for having an alliance with Gulen in the early years of his political career.

Almost 26,000 suspects have now been rounded up after the coup, which Ankara blames on followers of Gulen who built up a presence in key institutions including the military. Gulen denies the accusations.

Jagland's comments accepting the need for a crackdown contrasted with the tone of several EU officials who while condemning the coup have expressed alarm over the scope of the arrests.

"I recognise that of course there is a need for taking on those who were behind this coup and also on this secret network," Jagland said after talks with Foreign Minister Mevlut Cavusoglu in Ankara.

"I would like to say there has been too little understanding from Europe over what challenges this has caused to the democratic and state institutions of Turkey," said Jagland, referring to Gulen's group.

"We however have been informed about it for a very long time. So therefore of course we see a need for cleaning up all of this," added Jagland, one of the most senior European officials to visit Turkey in the wake of the botched July 15 putsch.

- 'Intolerable' -

According to Interior Minister Efkan Ala, 25,917 people have now been detained, 13,419 of whom have been remanded in custody over their roles in the coup.

The CoE promotes democracy and the rule of law in Europe and its members include states who are not EU states like Turkey and also Russia.

But Jagland, who was later to meet President Recep Tayyip Erdogan, also emphasised the importance of all moves being carried out within the rule of law and the European Convention on Human Rights.

Turkey has declared a three-month state of emergency after the coup and said it will suspend the rights convention for this period.

Cavusoglu said: "We have never made compromises on our understanding of democracy and will never do."

Later the foreign minister took to Twitter to praise Jagland's stance, saying he hoped the solidarity of the CoE chief "sets an example for our other European friends."

"Europe should realise how it moves away from its own values as it excludes Turkey," Cavusoglu said.

Turkey has sent an array of documentation to the United States asking for Gulen's extradition and has so far expressed exasperation over the slowness of Washington in taking up the issue.

"You have to be blind and deaf not to understand that he is behind all of this," Erdogan said in an interview with Mexican television, describing any delay in extradition as "intolerable".

- 'Forgive us' -

Gulen's influence in Turkey goes back to the premierships of Bulent Ecevit in the 1970s and he was a strong presence in Turkish politics before anyone had even heard of Erdogan.

As a new and untested forces in politics, Erdogan's Justice and Development Party (AKP) was happy to team up with Gulen after it came to power in 2002, sharing a conviction that Islam should lie at the heart of politics.

But in his first apology on the issue, Erdogan said he had failed to see the "true face" of his former ally.

"I know that we have to give account both to our God and to our people. Let my God and my people forgive us," he said.

Turkish intelligence services have identified almost 56,000 alleged Gulen followers through years of intercepted messages on the app ByLock, a Turkish official told AFP.

"Decryption work is continuing," he said.

The coup was led by disgruntled elements in the military who the authorities say were followers of Gulen fast-tracked to senior positions by rigging in examinations.

In a symbolic sign of how he has regained control of the military, Erdogan on Wednesday visited the chief of staff headquarters for the first time since the putsch.