"We must prioritise security, and let's start by building a common European army," Hungary's rightwing prime minister, Viktor Orban, said at talks with Czech, German, Polish and Slovak leaders.
The five-nation gathering in Warsaw is part of a string of meetings among various groups of countries ahead of a summit on the EU's future following the June 23 British referendum.
Leftist Czech Premier Bohuslav Sobotka, for his part, said that "we should also begin a discussion about creating a common European army."
German Chancellor Angela Merkel also supported the idea of stronger security but urged caution on how plans were translated into acts.
"Security is a fundamental issue... we can do more together in the areas of security and defence," she said.
"Brexit is not just any event, it's a breaking point in the history of EU so we need to work out a very careful response," Merkel added, according to the official English translation of her words.
In an early response to Britain's shock vote to exit the EU, Poland's powerful rightwing leader Jaroslaw Kaczynski called for EU institutional reforms that would forge a confederation of nation states under a president in charge of a powerful common military.
Challenge to EU
However, the concept of a common army is a thorny issue within the European Union (EU).
All five EU countries at Friday's Warsaw talks are also members of the 28-member NATO Western defence alliance.
But six of the EU's 27 post-Brexit membership do not belong to NATO: Austria, Cyprus, Finland, Ireland, Malta and Sweden.
EU and NATO ties with Russia plunged to their lowest point since the Cold War after Moscow's 2014 annexation of the Crimean peninsula from Ukraine.
The Kremlin's sabre-rattling in the Baltic region has also spooked NATO and EU members there.
EU leaders from 27 states meet on September 16 in the Slovak capital of Bratislava for an informal summit that will go ahead without Britain.
Talks are likely to be challenging as Berlin's preferred vision of a centralised, federal Europe clashes with proposals for a confederation of nation states popular among leaders of eastern EU members.
Migration policy is another highly controversial issue on the agenda, with eastern members the Czech Republic, Hungary, Poland and Slovakia refusing to take in refugees under an EU-wide quota system championed by Berlin.
Merkel insisted on Friday that remaining members must focus on strengthening the bloc through boosting the economy and "finding common ground" despite divisions over issues like migration policy.
"People will only accept Europe if it will ensure their (economic) wellbeing. We must be leaders in the area of technology. We want to offer well-paid jobs. We have a lot to do," Merkel added.
Poland on Friday urged leaders to focus on economic growth and address a perceived lack of democracy in European institutions.
Rightwing Prime Minister Beata Szydlo urged "reforms that Europeans expect allowing the European Union to become a stronger, development-oriented EU - but above all for Europeans to feel they are in charge of the European Union."