European Commission President Ursula von der Leyen has announced legislation to remove more migrants without the right to stay in the European Union.

"The EU's migration policy can only be sustainable if those who do not have the right to stay in the EU are effectively returned," von der Leyen wrote in a letter sent to capitals.

"However, only around 20% of third-country nationals ordered to leave have actually returned."

The new law is to define the obligations of returnees and streamline the return process, she wrote.

Drafting the proposal is to be a priority for the new migration commissioner who is expected to take office at the end of the year.

In the letter, sent ahead of an EU leaders' summit in Brussels starting on Thursday, von der Leyen also raised concerns about people being displaced by the conflict in the Middle East.

More than 1 million people have been displaced in recent weeks "with a clear potential for further international displacement," she wrote, urging EU leaders "to work on contingency planning."

Migration is likely to dominate the upcoming EU summit after Poland recently announced plans to temporarily suspend the right to asylum.

Warsaw accuses Russian President Vladimir Putin and Belarusian leader Alexander Lukashenko of pushing migrants to the Polish border in order to destabilize the 27-member EU and undermine security.

"We need a clear and determined European response to counter these activities, while not allowing Russia and Belarus to use our own values against us," von der Leyen wrote to EU leaders.

The commission president also indirectly criticised border controls within the bloc's visa-free travel zone, the Schengen area, currently carried out by Germany.

"The reimposition of border controls should be a measure of last resort," she wrote.