German Chancellor Angela Merkel has declared the EU is in a "critical situation" as European leaders gathered for a post-Brexit summit in Slovakia.

"We need solutions for Europe and we are in a critical situation," Mrs Merkel said as she arrived at the gathering.

"You can't solve all Europe's problems in one summit. What we have to do is show in our deeds we can do things better in the realms of security and fighting terrorism, and in the field of defence."

It comes as leaders of the 27-member EU gathered in Bratislava, Slovakia, to discuss the union's future. Britain was not represented at the meeting.

EU members are currently deeply divided on how to stimulate economic growth, deal with the fallout from Brexit, and cope with the refugee crisis.

Donald Tusk, president of the EU Council, urged leaders to take a "sober and brutally honest" look at the problems facing the bloc.

"We haven't come to Bratislava to comfort each other or even worse to deny the real challenges we face in this particular moment in the history of our community after the vote in the UK," he said.

In a letter inviting EU leaders to the summit, he said that the "failures" of British politicians was responsible for Brexit.

Leaders had a responsibility to the EU, he said: "It also means refraining from the constant accusations aimed at the Union, which sometimes are justified, but more often than not they serve as an easy excuse for one's own failures.

"This was also one of the reasons behind the Brexit vote."

Eastern and central European states, including Hungary and hosts Slovakia, oppose an EU scheme to resettle 160,000 refugees across member states. Slovak Prime Minister Robert Fico has said his country will not accept "one single Muslim migrant" and has mounted a legal challenge to the scheme.

In a sign of tensions between member states, Luxembourg has called for Hungary to be expelled from the EU for treating refugees "worse than wild animals".

The meeting is one of a series that will be held before the 60th anniversary of the EU in Rome in March. Leaders plan to form a 'roadmap' of policy commitments to be ratified at the summit.