A European Union leaders' summit kicked off in Brussels on Thursday with the ongoing Brexit talks the highlight of the first day.
Attending the European Council, NATO Secretary General Jens Stoltenberg said that Brexit would not change the U.K.’s relationship to NATO.
"Brexit just highlights the importance of stronger NATO-EU cooperation, and therefore I welcome that we have been able to lift NATO-EU cooperation up to a new level," he said.
Mentioning the EU's new defense initiative -- the Permanent Structured Cooperation, or PESCO -- the NATO chief said that cooperation had reached "a new level."
Stoltenberg also warned that non-EU allies are key to European security as 80 percent of NATO’s defense expenditures will come from them after Brexit next year.
"It also highlights the importance of strengthening EU-NATO cooperation," he added.
May disappointed by defeat in parliament
British Prime Minister Theresa May said she was "disappointed" that members of her Conservative party on Wednesday opposed giving parliament final say on Brexit.
The majority of British MPs favored an amendment to a much-debated EU Withdrawal Bill giving parliament the legal guarantee of a vote on the final Brexit deal.
"But the EU withdrawal bill is making good progress through the House of Commons and we're on course to deliver on Brexit," May said.
Dutch Prime Minister Mark Rutte called for "a legally binding text" that will transform the U.K.'s initial terms of withdrawal into a binding text "as soon as possible."
European Council President Donald Tusk also addressd Brexit on his way to the summit, saying: “I have no doubt that the real test of our unity will be the second phase of Brexit talks."
He also said there is a different mentality between eastern and western Europe when it comes to relocating migrants from frontline countries.
He called the EU's compulsory refugee quota "ineffective" and "highly divisive".
- Merkel blasts Tusk’s remarks
German Chancellor Angela Merkel criticized Tusk’s remarks, saying: "We need solidarity not just in regulating and steering migration on the external borders but we also need internal solidarity."
She added that the EU member states cannot show "selective solidarity" on issues the bloc deals with.
Merkel also said that the eurozone needs to boost its international competitiveness and that member states' economies must be harmonized with each other.
"We will take necessary decisions to further strengthen the eurozone," she added.