US Secretary of State John Kerry and High Representative of the European Union for Foreign Affairs and Security Policy Federica Mogherini in Brussels | John Thys/AFP via Getty Images Kerry, Mogherini promise EU-NATO ‘scrutiny’ of Turkish actions after coup US secretary of state and EU foreign policy chief warn Ankara to stick to rule of law.

The EU and U.S. united Monday in calling for Turkey to uphold democratic principles as it responds to a failed coup attempt over the weekend, and said they would increase their vigilance over Recep Tayyip Erdoğan's government in the days ahead.

Speaking after a meeting with the EU's 28 foreign ministers, U.S. Secretary of State John Kerry urged the Turkish government to “maintain calm and stability throughout the country,” and “to uphold the highest standards of respect for [the nation’s] democratic institutions and the rule of law.”

Kerry and the EU's foreign policy chief, Federica Mogherini, both said they were concerned about the Turkish government's crackdown on the alleged coup plotters — which included thousands of arrests of officials and military leaders and the possibility, raised by Erdoğan over the weekend, that the country would reinstate the death penalty.

Kerry also warned that the NATO alliance, of which Turkey is a member, "has a requirement with respect to democracy and NATO will indeed measure very carefully what is happening."

“A lot of people have been arrested and arrested very quickly,” Kerry told reporters. “The level of vigilance and scrutiny is going to be very significant in the days ahead.”

Kerry said he had spoken several times over the weekend with Turkey's foreign minister, and had been assured the Turkish government would “move in ways” to respect the democratic process. Asked about Erdoğan’s comments on the death penalty, Mogherini said its reinstatement would have serious consequences for Turkey's bid to join the European Union. "No country can become an EU member state if it introduces death penalty,” she said.

She also said the EU called for “the full observance of Turkey’s constitutional order and rule of law.”

Kerry was in Brussels after visits to Paris, Moscow and Luxembourg, to join a meeting of EU foreign ministers. The agenda for their working breakfast also included discussions on the Middle East peace process, the global response to the Bastille Day attack in Nice and terrorism in general, and relations with Russia.

Earlier in the day, top EU officials and diplomats expressed similar concerns about the Turkish government’s reaction to the coup over the weekend, and also insisted that Ankara respect the rule of law.

According to Turkish authorities, 290 people died during violent clashes over the weekend in Turkey.