EU leaders on Thursday talked over possible sanctions should Russia take further military action in Ukraine, at an impromptu summit held in Brussels.

European Council President Charles Michel tweeted that he spoke with Ukrainian President Volodymyr Zelensky just before chairing the summit, which was more an information and coordination session than one resulting in any decisions.

"Closely following the situation on the ground. Deescalation is urgently needed," Michel said, reiterating EU "solidarity" with Ukraine.

European Commission chief Ursula von der Leyen addressed the informal summit, which was announced just a day earlier, to give only a brief outline of potential sanctions the EU could impose on Russia. 

Participants emphasised that the European Union is ready to act swiftly if Russia further invades or attacks Ukraine.

"The important thing is that, in case of invasion, the West is ready to respond in a necessary and robust way so it can be seen that another country cannot be invaded without big costs being imposed," Danish Prime Minister Mette Frederiksen said.

Finland's Prime Minister Sanna Marin said there was still hope that a diplomatic solution would prevail. "But of course we are at the same time preparing very heavy, very massive sanctions," she said.

A commission spokesman stressed that any EU sanctions would be "reactive", in response to whatever action Russia might take, so they were not publicly defined at this stage.

The EU, like the US, is treating with caution Russia's declarations that it is pulling back troops from a massive deployment it has sent to Ukraine's borders.

"These events that seemed to announce a de-escalation are currently not being taken seriously and therefore we must remain prepared for any eventuality," Italian Prime Minister Mario Draghi said before joining the summit.

He said the EU needed "to keep our deterrence strategy firm, show no weaknesses".

He also said "the goal is to get President (Vladimir) Putin and President Zelensky to sit around the same table" in existing formats involving European powers to negotiate a peaceful outcome.

An EU spokesman, Peter Stano, told journalists that the commission was keeping a lid on possible sanctions. 

"Before anything happens we cannot discuss sanctions because we will be just speculating about the potential crime to which we are going to react," he said.

After their impromptu meeting, EU leaders were to begin a two -ay summit with African counterparts, seeking to reboot ties with pledges of major investment in the face of competition from China and Russia.