Ukraine conflict: What is Nato and how has it responded to Russia's invasion?

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.