Jean-Claude Juncker, the former Luxembourg premier who wants to be next head of the European Commission, said Europe must rush through an association deal with Moldova to avoid it becoming Russian President Vladimir Putin's next target for annexation.

"After events in Ukraine, it is of great urgency that the Europeans sign the association agreement with Moldova very quickly, in the next few weeks," Juncker told the German newspaper Die Welt am Sonntag in an interview.

The president of Moldova, an ex-Soviet republic of 3.5 million people, warned Russia this week not to consider any move to take over Moldova's separatist Transdniestria region in the same way it has taken control of Crimea in Ukraine.

He was responding to comments by the speaker of the region's separatist parliament urging Moscow to incorporate the mainly Russian-speaking region, which split from Moldova in 1990, a year before the dissolution of the Soviet Union.

"We have to steal a march on Putin, he has to know that he cannot do in Moldova what he did in Crimea," said Juncker.

"Otherwise Moldova could be the next victim of Russian aggression. Putin only reacts to EU action when we make it clear that we won't accept his behaviour," said Juncker, the leading candidate of the center-right European People's Party for the European Parliament elections in May.

Moldova initialed an agreement to strengthen ties with the EU late last year, which is half-way to signing, and it hopes to complete that process by the end of this year.

The EU signed a political association agreement with Ukraine this week to underscore support for the transitional government in Kiev, which Russia denounces as illegal. This will be followed by a trade agreement after planned Ukrainian elections.

It was the decision of Ukraine's former president Viktor Yanukovych to reject the association agreement with the EU and to seek closer ties with Russia last November that triggered the street protests that culminated in his ouster last month.

In response to the toppling of Yanukovych, Russia annexed Ukraine's Black Sea peninsula of Crimea, saying it had a duty to protect the region's ethnic Russian majority population. On Friday, Putin signed the laws completing the annexation.

Moldova, one of Europe's poorest countries, has had pro-Western governments since 2009 and has a difficult relationship with Russia, the source of most of its oil and gas.