The United States and Europe pressed Turkey on Monday to follow the rule of law and maintain democratic principles despite a crackdown after an attempted coup.

European Union foreign policy chief Federica Mogherini suggested that Turkey’s bid for E.U. membership could be at stake if it reinstates the death penalty against coup plotters.

“No country can become a partner state if it introduces the death penalty,” she told reporters.” That is key.”

Secretary of State John F. Kerry followed her warning, saying NATO will be scrutinizing Turkey in coming days to ensure that it fulfills the alliance's requirement that members adhere to democratic governance.

“NATO also has a requirement with respect to democracy,” Kerry said.

What we know about the failed coup attempt in Turkey

Kerry said Turkish Foreign Minister Mevlut Cavusoglu has repeatedly assured him that the government will respect democracy and the law, and he warned that NATO will “measure” Turkey’s 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.”

While NATO will be watching the Turkish government’s actions carefully, State Department spokesman John Kirby said, “it's too soon to say that their membership is at risk.”

Western governments are balancing support for the democratically elected government of Turkey against a military coup with growing concern that it is using the attempted overthrow to crack down in undemocratic ways.

“We expect the government will live up to the democratic principles enshrined in its constitution,” Kirby said. “International institutions like the E.U. and NATO will certainly be watching as events unfold because there are democratic responsibilities that come with membership.”

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 E.U., 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 E.U. 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 E.U. could be “destroyed” if Erdogan overreaches.

Following a breakfast meeting with the E.U. 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 U.S. 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 E.U. 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."