PARIS: France on Thursday expressed fear that the worst phase of Russia's war on Ukraine could be yet to start, with key Ukrainian cities at risk of encirclement by Russian forces.

"I think it is possible that the worst is ahead of us," Foreign Minister Jean-Yves Le Drian told France 2 TV, saying that Russia would look to besiege cities after its initial expectations of a quick victory were scuppered by Ukrainian resistance.

"The Russians entered Ukraine looking for a blitzkrieg, a fast advance that would allow Russia to control Ukraine," said Le Drian.

"It was not the case, due to the exemplary, courageous and strong resistance from the Ukrainians."

He said after the fall to Russia of the southern city of Kherson -- confirmed by Ukraine overnight -- centres including Kharkiv in the east, Mariupol in the south and Kyiv itself were at risk of encirclement.

"As fores build up around those cities we can fear a siege mentality," he said.

Le Drian recalled past Russian tactics in the war in Syria and also against separatists in the region of Chechnya.

"The Russians are used to this -- remember Aleppo, Grozny. This could be very grave. The disaster continues and Russia's despicable aggression continues."

Asked what President Vladimir Putin's strategy was for Ukraine, Le Drian replied the Russian leader wanted "the negation of Ukraine and the negation of a democratic country on his doorstep."

He said that the tensions over the pro-Moscow Ukrainian breakaway regions of Donetsk and Lugansk ahead of the Russian invasion on February 24 were just an "alibi, a pretext".

"Putin's wish is to avoid having democratic models on his doorstep that could eventually have an influence on the development of Russia," he said.

Le Drian is due later Thursday to visit Ukraine's neighbour Moldova in a show of support for its pro-EU government as Russian forces seek to press further east.