Ukraine conflict: What is Nato and how has it responded to Russia's invasion?
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Leaders of the Nato military alliance have approved major increases of forces in Eastern Europe.
They also said that any use of chemical or biological weapons by Russia would "result in severe consequences".
What is Nato?
Nato - the North Atlantic Treaty Organization - is a military alliance formed in 1949 by 12 countries, including the US, Canada, the UK and France.
Its aim was originally to counter the threat of post-war Russian expansion in Europe.
Soviet Russia responded 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 switched sides and joined Nato. The alliance now has 30 members.
Members agree to come to one another's aid in the event of an armed attack against any individual member state.
Why isn't Nato sending troops to Ukraine?
Because Ukraine isn't a Nato member, the alliance isn't obliged to come to the country's defence.
And Nato countries fear that if their troops confront Russian forces in Ukraine, it could lead to an all-out conflict between Russia and the West.
Nato says its members are "determined to do all we can to support Ukraine," but must ensure the war doesn't escalate beyond its borders.
This is why Nato rejected imposing a no-fly zone over Ukraine.
However, it has approved sending four new battlegroups to Slovakia, Hungary, Bulgaria and Romania - three of which border Ukraine - doubling Nato's presence in the region.
Western leaders meet to discuss Ukraine support
UK troops will not fight against Russia - Wallace
Why isn't Ukraine a Nato member?
Nato offered Ukraine a path towards membership in 2008. After Russia's annexation of Crimea in 2014, Ukraine made joining a priority.
But this hasn't happened, mainly because of Russia's long-standing opposition.
One of Russia's demands before the invasion was that Ukraine should never be allowed to join - something the alliance refused to support.
Russia fears Nato is encroaching on its territory by taking on new members from eastern Europe, and that admitting Ukraine would bring its forces into its backyard.
Ukraine's President Zelensky has accepted his country can't join Nato at present, saying: "It is clear that Ukraine is not a member of Nato. We understand this."
China joins Russia in opposing Nato expansion
Which weapons have the UK and other countries sent to Ukraine?
Ahead of the Nato meeting in Brussels, the UK said it would provide 6,000 new defensive missiles and £25m to help Ukraine pay its armed forces.
The UK has already supplied more than 4,000 shoulder-mounted NLAW anti-tank missiles, as well as a small number of Javelin missiles. It has also sent some Starstreak missiles.
A Ukrainian soldier holding a Javelin anti-tank missile system during exercises in 2021
It has also given body armour, helmets and boots.
In addition, the UK has deployed Sky Sabre, a long-range air defence system to Poland, along with 100 military personnel to operate it.
The US initially sent $200m (£152m) and then a further $350m (£267m) of weapons. This included Javelin missiles, Stinger anti-aircraft missiles and body armour.
The US has since announced a $1bn (£760m) security aid package. This includes more sophisticated longer-range weapons and drones.
It has also allowed other Nato countries to supply Ukraine with US-made weapons.
Nato has stepped up its military defences in eastern Europe
The EU has said it will spend up to 450m euros (£376m) to fund the supply of weapons to Ukraine. It is the first time in its history that the EU has helped provide arms for a warzone.
Germany, too, has dropped its long-standing restrictions on supplying weapons to a combat area.
The Netherlands, Belgium, Poland, Estonia, Latvia, the Czech Republic and Slovakia have also sent military equipment, including ammunition, fuel, and rations.
Are arms shipments to Ukraine making a difference?
How many troops does Nato have in Eastern Europe?
Even before the latest deployments are agreed, Nato already had troops stretching from the Baltic republics in the north to Romania in the south.
They were stationed there in 2014, after Russia's annexation of Crimea and are designed to act as a "tripwire" in case of a Russian attack.
Nato has sent elements of its 40,000-strong Response Force to Eastern European countries bordering Russia and Ukraine.
It has 100 fighter jets on high alert and 120 ships, including three carrier groups, patrolling the seas from the far north to the eastern Mediterranean.
The US has committed to sending more troops to Europe - they will join the four multinational battlegroups Nato has in Estonia, Latvia, Lithuania and Poland, and its multinational brigade in Romania.