In April, Ukrainian President Volodymyr Zelensky asked the military bloc to accelerate the ex-Soviet country's membership in the alliance, saying it was the only way to end fighting with pro-Russia separatists.

Zelensky's plea came after Russia amassed around 100,000 troops on Ukraine's borders, raising fears of a serious escalation in a years-long conflict with Moscow-backed separatists.

Speaking at a joint news conference with visiting OSCE Secretary General Helga Schmid, Foreign Minister Dmytro Kuleba said Kiev was grateful to NATO for its "open door" policy on Ukraine.

"But not a single step has been taken to implement this decision," Kuleba said.

Ukraine, which applied to join the US-led alliance in 2008, is pressing Western powers for more support as it seeks to deter any new aggression from Moscow.

But NATO members remain reluctant to embrace Ukraine as they want to avoid ratcheting up tensions with Russia.

Kuleba criticised the bloc's decision not to invite Ukraine to a June 14 summit in Brussels that US President Joe Biden is set to attend.

"We cannot understand how it is possible not to invite Ukraine, how it is possible not to find a format for Ukraine's participation in the summit this year," he said.

Kiev has repeatedly said it expects a NATO invitation to join a Membership Action Plan (MAP), the first formal step towards becoming a member.

Kuleba said that Ukraine and fellow ex-Soviet nation Georgia should receive such an invitation this year.

"It is difficult to imagine a better and more important moment," he said.

Ukraine's pro-Western leadership closely cooperates with NATO and in 2019 parliament adopted constitutional changes formally spelling out the country's aspirations to join the alliance and the European Union.