BRUSSELS (WASHINGTON POST) - United States Secretary of State John Kerry cautioned Monday (July 18) that Turkey's membership in Nato could be jeopardised if it abandons democratic principles and the rule of law in a post-coup crackdown.

"Nato also has a requirement with respect to democracy," Kerry told reporters after European Union foreign policy chief Federica Mogherini warned Turkey not to execute coup plotters. She noted that countries with the death penalty cannot join the European Union, as Turkey has sought to do.

Kerry said Turkish Foreign Minister Mevlut Cavusoglu has repeatedly assured him that the government will respect democracy and the law.

Kerry warned that Nato will "measure" its actions. "Obviously a lot of people have been arrested and arrested very quickly," Kerry said. "The level of vigilance and scrutiny is obviously going to be significant in the days ahead. Hopefully we can work in a constructive way that prevents a backsliding."

Friday's attempted coup in Turkey has turned what was expected to be a routine meeting of the European Council into crisis management.

Virtually every diplomat attending the meeting expressed concern, even alarm, over the Turkish government's arrests of thousands of judges and members of the armed forces in a purge that continued Monday.

Many Europeans fear that the crackdown could unleash a new wave of refugees fleeing persecution in Turkey. It also threatens a recent agreement in which Turkey agreed to take back some Syrian refugees, a policy aimed at reducing the number of Syrians crossing the Mediterranean to Greece. But the crackdown could prompt refugees to argue that they would not be protected in Turkey and should not be sent back.

For the EU, Turkey's position on the death penalty is a key indicator of human rights and rule of law.

On Sunday, President Recep Tayyip Erdogan told crowds of supporters demanding the death penalty for coup plotters that Turkey's parliament should consider reinstating the practice it abolished in 2004.

Virtually every diplomat attending the meeting issued a stern warning to Erdogan.

Austria's EU commissioner, Johannes Hahn, said Erdogan's crackdown "is exactly what we feared." He said the arrest of thousands of judges over the weekend looked "like something that had been prepared."

French Foreign Minister Jean-Marc Ayrault warned Erdogan against growing more "authoritarian". "We must be vigilant that Turkish authorities don't put in place a political system which turns against democracy," he told reporters.

And Jean Asselborn, the foreign minister of Luxembourg, said relations between Turkey and the EU could be "destroyed" if Erdogan overreaches.

Following a breakfast meeting with the EU diplomats, Kerry said the United States still has received no formal request for the extradition of Fethullah Gulen, a Turkish cleric residing in the United States whom Erdogan has publicly blamed for the attempted coup.

As he stated over the weekend, Kerry said the United States would consider a formal request but that it must meet US legal standards. On Monday, Kerry explicitly said Turkey must send "evidence", not allegations. "What we need is genuine evidence that withstands the standard of scrutiny that exists in many countries," he said.

"And if it meets that standard, there's no interest we have of standing in the way of appropriately honoring the treaty we have with Turkey with respect to extradition. We've never had such request. We've never had such evidence. We're doing nothing whatsoever to stand in way of legitimate process which respects the treaty."

Concerns over Turkey overshadowed the debut of Britain's new foreign secretary, Boris Johnson, on his first assignment abroad since last month's vote to leave the European Union led to a new government.

Johnson, a flamboyant politician who led opposition to EU membership, arrived late Sunday, after his Royal Air Force plane had to make an emergency landing to deal with a technical problem.

His fellow diplomats were expected to be warily sizing him up, since he has mocked many of them. But on arriving Monday, he adopted a conciliatory tone. "The message I'll be taking to our friends in the council is that we have to give effect to the will of the people and leave the European Union, but that in no sense means that we are leaving Europe," he told reporters. "We are not going to be in any way abandoning our leading role in European cooperation and participation of all kinds."