Ukraine's President, Volodymyr Zelenskyy, was asked in February last year what his first question to the newly inaugurated US President Joe Biden would be.
In response, he said: "Ukraine is not just saying in words that it wants to be an equal member of the alliance, an equal member of NATO.
"Putting away these phrases that we will all contemplate and communicate, the first simple question from me would be: 'Mr. President, why are we not in NATO yet?'" he said.
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What does NATO and Ukraine's current relationship look like?
NATO is present in Ukraine, and the country is both a security donor and recipient in its region.
The alliance provides support to Ukraine in multiple areas, including building capabilities and interoperability, cyber defence, Explosive Ordnance Disposal (EOD) and Countering Improvised Explosive Devices (C-IED).
NATO has supported Ukraine in the country's conflict with Russia, as has the UK.
Operational Orbital, the codename given to the UK Armed Forces' deployment in Ukraine, was started after the illegal annexation by Russia of Crimea, a peninsula, officially part of Ukraine, located on the north coast of the Black Sea.
Watch: US watching Russian presence near Ukraine 'very closely'.
Since 2015, British troops have been training and building the capacity of the Ukrainian Armed Forces and in November 2019, UK Defence Secretary Ben Wallace announced Operation Orbital would extend to 2023.
The training that UK forces provide focuses on basic infantry and medic skills, including medical, logistics, Counter Improvised Explosive Devices (C-IED), leadership and infantry skills, planning and maritime capacity building – diving, firefighting, damage control and sea surveillance.
The British and Ukrainian militaries also regularly collaborate in joint training exercises across the sea, land and air.
This training is key to the Ukrainian armed forces' focus to modernise and develop as they face Russian threats in the east of their country.
In September 2020, Mr Wallace announced the UK would lead a multi-national Maritime Training Initiative for the Ukrainian Navy.
This will see courses delivered by the Royal Navy and naval personnel from Sweden, Canada and Denmark in a range of areas.
At the 2016 Summit in Warsaw, NATO's support measures for Ukraine formed part of the Comprehensive Assistance Package (CAP).
This is shaped to help the country provide its own security, as well as put into place reforms in the security and defence sector.
The country has also successfully defended itself, and the wider Baltic and Black Seas region, against Russia for roughly seven years.
This has seen Ukraine gain experience in countering Russia in both traditional and hybrid warfare.
How close is Ukraine to joining NATO and what does it still need to do?
All of this would appear to make Ukraine a strong candidate to join NATO, and there are processes and plans in place.
There is a NATO-Ukraine Commission (NUC), providing a platform for the NATO Allies and Ukraine to communicate on shared security concerns.
In 2009, the Declaration to Complement the NATO-Ukraine Charter was signed – giving the NUC a key role in assisting the reform efforts of Ukraine.
Central to this process is the Annual National Programme (ANP), which looks to consider Ukraine's reform.
It is focused on key areas: political and economic issues; defence and military issues; resources; security issues; and legal issues.
Watch: Retired Lieutenant General Sean MacFarland discusses whether NATO is able to deal with Russia.
Allies assess the progress Ukraine has made under the ANP annually and, according to NATO, "the responsibility for implementation falls primarily on Ukraine".
In June 2020, NATO recognised Ukraine as an Enhanced Opportunities Partner – which will see the country helped to sustain contributions to the alliance.
It also includes increased access to NATO interoperability programmes and exercises, as well as more shared information.
Why has Ukraine not yet joined NATO?
For Ukraine to join the alliance, it must continue with Euro-Atlantic reforms, but the country has still not been issued with a Membership Action Plan (MAP).
The MAP is a programme of support designed to the needs of a particular country wishing to join NATO.
Countries taking part in the submission of the MAP must hand in individual annual national programmes on how they are preparing for future membership – covering political, economic, defence, resource, security and legal aspects.
At a summit in Bucharest in 2008, NATO made clear that Ukraine would become a member of the alliance at some point and supported the country's application for a MAP – agreeing this would be the next step towards membership.
It also outlined the same path for Georgia.
Watch: NATO condemns 'unjustified' Russian activity near Ukraine in April 2021.
Both countries, however, are still waiting for the MAP and have since been involved in conflicts with Russia.
In April 2021, the European Union estimated 100,000 troops had gathered near Ukraine's border – with only "a spark" needed to start a confrontation.
NATO responded by calling for the exercises to come to an end, with NATO Secretary General Jens Stoltenberg calling Russia's actions "unjustified, unexplained and deeply concerning".
Olha Stefanishyna, Ukraine's Deputy Prime Minister for European and Euro-Atlantic Integration, said avoiding Ukraine for a MAP was giving "not a very good signal to the Kremlin".
In February 2021, Ukrainian Prime Minister Denys Shmyhal visited NATO headquarters and discussed Ukraine's next steps towards Euro-Atlantic integration – with Mr Shmyhal outlining that Ukraine's goal is NATO membership.
Some believe Ukraine joining NATO would provoke Russia as the alliance moves further east.
Watch: Exercise Rapid Trident: NATO troops test vital combat skills in Ukraine.
A hardware deal announced in November 2021 is set to see British warships and missiles sold to Ukraine, though a month later reports suggested the UK and its allies were 'highly unlikely' to send troops to defend Ukraine were a Russian invasion to take place.
The deal was signed on-board HMS Defender in Ukraine, but the vessel, on its return to the Carrier Strike Group, was shadowed by Russian vessels and buzzed by jets as it sailed through the disputed waters around Crimea.
Russia also threatened to "drop bombs... right on target" if Royal Navy vessels sail through waters around Crimea again.
The UN believes there have been some 13,000 deaths since the beginning of the conflict in the east of Ukraine in 2014.
In December 2021, it calculated roughly 1.5 million people have been internally displaced and were living permanently in government-controlled areas of Ukraine.
President Biden has warned Vladimir Putin over a phone call that America could impose new sanctions against Russia if it takes further military action against Ukraine.