Ukrainian President Volodymyr Zelensky on Tuesday voiced frustration over NATO reluctance to speed up Kiev's membership in the alliance, as Russia presses to stop the military alliance from expanding eastwards. 

Kiev has for years been seeking to join the US-led alliance, but Western officials have on numerous occasions said the prospect is not in the cards anytime soon, frustrating Ukraine.  

"We cannot accept the theory that is now very popular about (Ukraine joining) the EU in 30 years and NATO sometime in about 50 years. It demotivates us and slows us down," Zelensky said during a meeting with Ukrainian ambassadors. 

He said Ukraine wants to "get a very clear timeline" from NATO on the prospect of membership

"And we want to get it in 2022," Zelensky said. 

Russia has said that any moves towards Ukraine joining NATO would be a red line for Moscow.

The Kremlin has submitted proposals to contain the United States and NATO in the former Soviet Union and Eastern Europe, calling for urgent negotiations with Washington as it amasses forces near Ukraine.

Russia has also massed some 100,000 troops near Ukraine's borders, with fears growing that it could be preparing a large-scale attack. 

Moscow annexed Ukraine's Crimea peninsula in 2014, and backs separatists fighting Kiev in the east of the country. 

Zelensky said Tuesday Kiev wants to "return" Crimea and "achieve EU membership in the coming years."

Western countries have warned Moscow of massive economic sanctions should its troops launch an attack. 

They also have rejected Russian calls for Ukraine to be excluded from future membership in the alliance.

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.

Ukraine has been fighting pro-Russia separatists in the eastern Donetsk and Lugansk regions since 2014.

The fighting has claimed more than 13,000 lives.