Ukraine: What is Nato and why doesn't Russia trust it?
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Members of Nato are weighing up what they should do in the event that Russia invades Ukraine.
The alliance - which includes the US, the UK, France and Germany - has moved troops to the Russian border and some nations have given military aid to Ukraine.
What is Nato?
Nato - the North Atlantic Treaty Organisation - is a military alliance formed in 1949 by 12 countries, including the US, Canada, the UK and France.
Members agree to come to one another's aid in the event of an armed attack against any one member state.
Its aim was originally to counter the threat of post-war Russian expansion in Europe.
In 1955 Soviet Russia responded to Nato by creating its own military alliance of Eastern European communist countries, called the Warsaw Pact.
Following the collapse of the Soviet Union in 1991, a number of former Warsaw Pact countries became Nato members. The alliance now has 30 members.
Is Russia preparing to invade Ukraine?
China joins Russia in opposing Nato expansion
What is Russia's current issue with Nato and Ukraine?
Ukraine is a former Soviet republic bordering both Russia and the EU.
It is not a Nato member, but it is a "partner country". This means there is an understanding that Ukraine may be allowed to join the alliance sometime in the future.
Russia wants assurances from Western powers that this will never happen.
However, the US and its allies are refusing to bar Ukraine from Nato, saying that as a sovereign nation it should be free to decide on its own security alliances.
Ukraine's ambassador to the UK, Vadym Prystaik, suggested recently that Ukraine was willing to be "flexible" over its goal to join the Western military alliance. However, a spokesman for President Volodymyr Zelensky contradicted him, saying Ukraine remains committed to its aspirations to join Nato and the EU as set out in in its constitution.
Ukraine has a large population of ethnic Russians and close social and cultural ties to Russia. Strategically, the Kremlin sees it as Russia's backyard, and Russian president Vladimir Putin has written that Ukrainians and Russians are "one people".
What else is Russia concerned about?
President Putin claims Western powers are using the alliance to encroach on Russia, and he wants Nato to cease its military activities in eastern Europe.
He has long argued the US broke a guarantee it made in 1990 that Nato would not expand eastwards.
Nato rejects Russia's claims and says only a small number of its member states share borders with Russia, and that it is a defensive alliance.
What has Nato done in the past about Russia and Ukraine?
When Ukrainians deposed their pro-Russian president in early 2014, Russia annexed Ukraine's southern Crimean peninsula. It also backed pro-Russian separatists who captured large swathes of eastern Ukraine.
Nato did not intervene, but it did respond by placing allied troops in several eastern European countries for the first time. These forces are designed to be a "tripwire" in case Russia invades Nato territory.
Since Russia's annexation of Crimea, Nato has put battle groups in eastern Europe
It has four multinational battalion-size battlegroups in Estonia, Latvia, Lithuania and Poland, and a multinational brigade in Romania.
It has also expanded its air policing in the Baltic states and eastern Europe to intercept any Russian aircraft which breach the borders of member states.
Russia has said it wants these forces out.
What commitments has Nato given Ukraine?
The United States has sent nearly 3,000 extra troops to Poland and Romania to reinforce Nato's eastern borders, and put another 8,500 combat-ready troops on alert (there are no plans to deploy any troops in Ukraine itself).
It has also sent weapons worth $200m including Javelin anti-tank missiles and Stinger anti-aircraft missiles, and allowed other Nato countries to supply US-made weapons to Ukraine.
The UK has supplied Ukraine with 2,000 short-range anti-tank missiles, sent 350 more troops to Poland and doubled its strength in Estonia with an extra 900 troops.
Nato has stepped up its military defences in eastern Europe
It has sent more RAF jets to southern Europe and a Royal Navy vessel to patrol the eastern Mediterranean alongside other Nato warships.
It has also ordered 1,000 troops to be in a state of readiness to provide support in the event of a humanitarian crisis caused by a Russian invasion of Ukraine.
Denmark, Spain, France and the Netherlands, have also sent fighter jets and warships to eastern Europe and the eastern Mediterranean.
There are plans for France to lead a Nato battlegroup in Romania and send troops there, but Nato says it will take several more weeks before the details are sorted out.