That could bring the former Soviet republic closer to NATO, anathema to its old overlord Moscow, which has massed tens of thousands of troops near Ukraine's borders in recent weeks.

British Prime Minister Boris Johnson is due in Kyiv later on Tuesday.

"I hope that in the near future we will be able to officially launch a new regional format of cooperation Ukraine-Poland-UK, in the context of ongoing Russian aggression, we should sign a trilateral document on cooperation to strengthen regional security," said Ukraine's Prime Minister Denys Shmygal.

Speaking at a joint news conference with Shmygal, his Polish counterpart Mateusz Morawiecki said Warsaw would help Ukraine with gas and arms supplies, as well as humanitarian and economic aid.

"Living close to a neighbour like Russia we have the feeling of living at the foot of a volcano," said Morawiecki, adding Warsaw would provide Ukraine with artillery ammunition, mortars, portable air-defence systems and surveillance drones.

Morawiecki called on Germany not to start the Nord Stream 2 pipeline, saying it posed grave security risks. He said standing up to Russia was not only important for Ukraine but for all of Europe and NATO.

"Through launching this pipeline, Berlin is loading Putin's pistol, which he can then use to blackmail the whole of Europe," he said, referring to Russian President Vladimir Putin.

Morawiecki said foreign ministers were "working on a potential format that could tighten cooperation on various fields between Poland, Ukraine and Britain."

Additional reporting by Natalia Zinets, writing by Matthias Williams, Gabriela Baczynska, Editing by William Maclean