Video report by ITV News Europe Editor James Mates
Turkey has been warned that restoring the death penalty to punish those involved in last Friday's failed coup would rule out future membership of the European Union.
President Recep Tayyip Erdogan said on Sunday he could not ignore calls for the death penalty to be imposed against those who attempted to overthrow the government.
Around 6,000 people have been arrested and nearly 8,000 police officers suspended.
Speaking at the European Council, EU Foreign Affairs representative Federica Mogherini said: "No country can become an EU member state if it introduces that penalty, that is very clear."
Meanwhile US Secretary of State John Kerry warned Turkey's NATO membership is at risk if the country abandons democracy.
"NATO has a requirement with respect to democracy and NATO will indeed measure very carefully what is happening," he said.
The summit in Brussels was Boris Johnson's first since being appointed Foreign Secretary in Theresa May's new-look cabinet.
Speaking to ITV News, Mr Johnson, who led the successful campaign to leave the EU, said: "Obviously I was making the crucial point from the UK's perspective, which is that we must keep effect the will of the people and Brexit must mean Brexit.
"But that in no sense means the end of Britain's commitment and participation in Europe and our support for all sorts of European adventures particularly on the foreign policy field."
The Foreign Secretary added that his reception at the summit had been "very good", but his French counterpart said Mr Johnson cut a "humble" figure, and failed to apologise for comparing the EU with Hitler.
Jean-Marc Ayrault, who branded Mr Johnson a liar last week over his referendum claims, said he did not take the Hitler remarks personally.
"No. He did not apologise," Mr Ayrault said.
"I did not feel it targeted me when he said that, nobody around the table did.
"Everyone knows what the EU is. Everyone knows, who discusses it reasonably, that the EU presents a great opportunity for freedom, democracy and for prosperity.
"So Boris Johnson, I would say, came to this council with some humility."