EU foreign affairs chief Federica Mogherini on Tuesday urged the bloc to do more in its own defence, just as leaders discussed Britain's leave vote which will result in the loss of a major military power.

"As Europeans we must take greater responsibility for our security. We must be ready and able to deter, respond to and protect ourselves against external threats," Mogherini said in a policy review submitted to European Union leaders meeting in Brussels.

They were due later Tuesday to hear Prime Minister David Cameron explain Thursday's Brexit vote and the next possible steps to give it effect.

While Britain is an active contributor to the currently very limited EU military operations, Cameron has consistently opposed any idea of a separate EU army.

Mogherini's review, a year in the making, lays down ambitious goals for the EU in the face of new security challenges in the east, highlighted by the Ukraine crisis, and in the south, driven by turmoil across Africa and the Middle East.

"While NATO exists to defend its members -- most of which are European -- from external attack, Europeans must be better equipped, trained and organised to contribute decisively to such collective efforts, as well as to act autonomously if and when necessary," the review said.

"An appropriate level of ambition and strategic autonomy is important for Europe's ability to foster peace and safeguard security within and beyond its borders."

EU leaders had tasked Mogherini with drawing up the "Global Strategy on Foreign and Security Policy" last year as Europe struggled to deal with its worst migrant crisis since World War II.

The report was meant to be a top agenda item at the EU summit on Tuesday and Wednesday in Brussels but the Brexit vote meant discussion was limited.

Britain would be a key element in any more ambitious EU security strategy, given its status as a nuclear-armed and veto-wielding permanent member of the UN Security Council.

Only France matches this standing while Germany, the most powerful EU member state, is very wary of any foreign military involvement.

Mogherini has consistently argued that the EU should play a global role in keeping with its status as one of the world's largest economies.

Some 22 out of the current 28 EU nations are also members of NATO and when it comes to security, they look first to the US-led alliance.