Von der Leyen pledges to help return Ukrainian children

"It's a horrible reminder of the darkest times of our history," she said. "It's something especially important to me," she added.

It was "an extraordinary act of barbarity", Polish prime minister Mateusz Morawiecki also said.

"The European Commission realises that Poland can be the best partner in publicising this issue internationally. We have also been asked for this by the Ukrainian side," he added.

Russia has taken some 16,200 Ukrainian children over the past 12 months as part of its campaign of terror, Ukraine says, just 300 of whom made it home so far.

The EU-Polish project will also rely on UN agencies to trace them and try to extract them from Russian adoptive families and foster homes.

The international conference, details of which are not yet public, will be "just the beginning of very, very hard work", von der Leyen said.

The EU was also preparing an 11th round of Russia sanctions, she confirmed.

"The 11th package [of sanctions] will mainly deal with the question of [sanctions] circumvention and how we can go against it," she said.

An EU special envoy was already touring third countries believed to be helping Russia get prohibited technology, she added.

But counter-circumvention aside, Estonia has also said the EU should target more Russian banks and oligarchs.

Finland says more Russian elite family members should be listed, while Lithuania wants to go after Russian nuclear energy firm Rosatom.

Existing EU sanctions already cover Russian officials involved in the child-abduction programme.

The International Criminal Court (ICC) in the Hague issued a personal arrest warrant for Russian president Vladimir Putin last week over the stolen children.

German authorities have said they would arrest Putin if he landed on their territory, prompting former Russian president Dmitry Medvedev to issue fresh nuclear threats against Berlin on Thursday.

But von der Leyen, who is a former German defence minister, appeared unfazed by Moscow's sabre-rattling following her EU meeting.

"This is a war crime," she said of Putin's child abductions. "These criminal actions completely justify the arrest warrant issued by the ICC," she said.