The United Nations Secretary General has said that he is "greatly concerned" over the latest flare-up in the Russia-Ukraine conflict.

Referring to the 25 November incident in the Black Sea involving Ukrainian and Russian vessels, António Guterres urged both countries to "exercise maximum restraint" and take steps to reduce tensions.

The European Union will next month consider further sanctions against Russia over the latest flare-up in the conflict, the foreign minister of Austria, which holds the rotating EU presidency, said today.

"On the issue of further sanctions, time will tell - we have a summit in December," Austrian foreign minister Karin Kneissl told reporters after talks with her German counterpart Heiko Maas.

"Everything depends on the accounts of events and the actions of both sides. But it will need to be reviewed."

Ms Kneissl said it was "one side's account versus the other's" in the latest tensions, in which Kiev declared martial law in response to Moscow's seizure of three of its navy vessels.

Thus the facts would need to be studied before conclusions could be drawn, she said.

Mr Maas said the latest escalation showed "that the annexation of Crimea ... is also a problem for the security of us all in Europe".

He repeated Berlin's call for Russia to release the seized ships and sailors "as soon as possible".

"Both sides must now do their part for deescalation," he added.

The United States and EU have already imposed sanctions on Russia over the Ukraine conflict.

Ukraine's Western allies have accused Russia of using force without justification in the naval confrontation, while Kiev urged its partners to impose further sanctions on Moscow.