Ukraine conflict: What is Nato and how has it responded to Russia's invasion?
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Ukraine's president Volodymyr Zelensky has acknowledged that his country cannot join Nato in the near future - despite its long-standing hope to do so.
Nato secretary-general Jens Stoltenberg said the security bloc remained united in its support of Ukraine. Member states including Britain are already sending weapons to help it fight the Russian invasion
However, the alliance has rejected calls to enforce a "no-fly zone", saying it could lead to direct conflict with Russia.
Russia has put its nuclear forces on "special alert" to counter what it calls Nato's aggression.
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.
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 switched sides and became Nato members. The alliance now has 30 members.
Why isn't Ukraine a Nato member?
Nato originally offered Ukraine eventual membership back in 2008, and after Russia's annexation of Crimea in 2014, Ukraine made joining Nato a priority.
But this hasn't happened, mainly because of Russia's long-standing opposition to such a move.
Russia fears Nato is encroaching on its territory by taking on new members in eastern Europe, and that admitting Ukraine would bring Nato forces into its backyard.
Ukraine's President Zelensky has now accepted his country cannot join Nato at present, saying: "It is clear that Ukraine is not a member of Nato. We understand this."
The UK Prime Minister Boris Johnson made the same point, saying, "There is no way Ukraine is going to join Nato any time soon".
One of Russia's demands before the invasion was that Ukraine should never be allowed to join - something the alliance had refused to support.
China joins Russia in opposing Nato expansion
Why isn't Nato sending troops to Ukraine?
Ukraine is not a member of Nato, so the alliance is not obliged to come to the country's defence.
Nato countries have also made it plain that if their troops were to confront Russian forces in Ukraine, it could lead to an all-out conflict between Russia and the west.
UK troops will not fight against Russia - Wallace
Why won't Nato impose a no-fly zone?
Nato has rejected imposing a no-fly zone over Ukraine, to deny Russia air superiority and protect civilians.
It wants to avoid conflict between Russian and its air forces.
Nato planes might need to fire anti-radiation missiles (ARMs) at radar installations on Russian territory.
Russia could use this as a reason to declare war on Nato countries.
How is the West helping Ukraine?
Several Nato nations have been sending weapons to Ukraine to help its defend itself against Russia.
The US has been the biggest supplier of military equipment. It initially sent $200m (£152m) and then a further $350m (£267m) of weapons. This included hand-held Javelin anti-tank missiles, Stinger anti-aircraft missiles and body armour.
The US has now expanded this and announced a $1bn (£760m) security aid package. This will include the deployment of more sophisticated longer-range weapons and drones to Ukraine,
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.
Britain, The Netherlands, Belgium, Poland, Estonia, Latvia, the Czech Republic and Slovakia are among those Nato members which are also sending military equipment - including anti-tank weapons, anti-aircraft missiles, ammunition, fuel, helmets, body armour and rations.
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Are arms shipments to Ukraine making a difference?
How many troops does Nato have in Eastern Europe?
Nato already has troops stretching from the Baltic republics in the north to Romania in the south and now deploying more forces.
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 is now sending 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.
"We will defend every ally and every inch of Nato territory," said alliance secretary-general, Jens Stoltenberg
President Putin of Russia has put the country's nuclear forces onto "special alert" because of "aggressive statements" by the West.
The announcement does not mean Russia intends to use the weapons.
The US has committed to sending more troops to Europe, but President Biden said they will not be fighting in Ukraine itself.
The extra troops will join the four multinational battlegroups Nato has in Estonia, Latvia, Lithuania and Poland, and its multinational brigade in Romania.