What is Nato, which countries are members and will they increase defence spending?
US President-elect Donald Trump has called on Nato's European members to spend 5% of their national incomes on defence.
That is more than double the military alliance's current target of 2%.
Nato secretary-general Mark Rutte has also told member countries to boost military spending as part of a new "wartime mindset".
What is Nato and why was it set up?
Nato - the North Atlantic Treaty Organization - was formed in Washington DC in 1949 by 12 countries: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the UK and the US.
Nato's primary purpose, external was to block expansion in Europe by the former Soviet Union - a group of communist republics which included Russia.
Members agree that if one of them is attacked, the others should help defend it., external
Nato does not have its own army, but member states can take collective military action in response to crises.
For instance, the body supported the UN by intervening in the war in the former Yugoslavia between 1992 and 2004.
It also co-ordinates military plans and carries out joint military exercises., external
Which countries are Nato members?
Nato has 32 members across Europe and North America - the original 12 founders plus 20 countries which have joined since 1949.
After the Soviet Union's collapse in 1991, many Eastern European countries became members, including Albania, Bulgaria, Hungary, Poland, the Czech Republic, Slovakia, Romania, Lithuania, Latvia and Estonia.
Finland - which has a 1,340km (832 mile) land border with Russia - joined in April 2023. Sweden became a member in March 2024.
Having been neutral for decades, both applied to Nato in May 2022, shortly after Russia invaded Ukraine.
Ukraine, Bosnia and Herzegovina, and Georgia have also asked to join.
Why isn't Ukraine a Nato member?
Russia has consistently opposed the idea of Ukraine becoming a member, fearing it would bring Nato forces too close to its borders.
However, in 2008, the alliance said that Ukraine could eventually join, external.
After Russia's invasion, Ukrainian President Volodymyr Zelensky asked for this process to be fast-tracked.
Although then Nato chief Jens Stoltenberg reconfirmed Ukraine could join "in the long term", he said this would not happen until after the war ended.
How much do Nato members spend on defence?
Nato currently asks every member country to spend at least 2% of national income - also known as GDP - on defence.
It is thought that 23 countries met that target in 2024, compared to only three in 2014.
Chart showing the growing number of Nato members meeting the 2% spending target, from three in 2014 to 23 in 2024. NB the figures for 2023 and 2024 are estimates.
The countries which spend the largest share of GDP on defence are the US and those close to Russia, such as Poland and the Baltic states.
In July 2024, Prime Minister Sir Keir Starmer said the UK will increase its defence spending to 2.5% of GDP.
In December 2024, Nato secretary general Rutte said that member states would have to "shift to a wartime mindset" and spend "considerably more than 2%" on defence, external.
During his first term as US president, Trump persuaded many Nato members to increase defence spending. In 2018 he said that all countries in the alliance should be committing 4% of GDP.
At a press conference at his Florida home in January 2025, Trump urged Nato's European members to spend 5%, external, telling reporters: "They can all afford it."
While campaigning for his second presidential term, Trump said he might encourage Russia to attack Nato countries which failed to spend enough, remarks which were strongly condemned by the White House and former Nato chief Stoltenberg.
How are Nato countries helping Ukraine?
Nato said Russia's invasion of Ukraine posed the "most significant and direct threat to allies' security"., external
The alliance has not sent troops to Ukraine or enforced a no-fly zone over the country for fear of being pulled into a direct conflict with Russia.
However, individual member states have supplied arms and equipment.
German research organisation the Kiel Institute, external said that the US had allocated 59.9bn euros (£49.7bn) for military support to Ukraine between February 2022 and October 2024.
European Nato members provided 52.6bn euros (£43.7bn) over the same period.
The US, UK, Germany and Turkey and others have sent anti-tank weapons, missile defence systems such as Patriot, artillery guns, tanks and drones.
The US, UK and France have also supplied long-range missiles such as Atacms and Storm Shadow/Scalp.
Atacms missiles can reach up to 300km (186 miles) and are hard to intercept due to their high speed
In August 2024, Ukraine received two US-made F16 fighter jets - the first of more than 60 pledged by Nato member states.
Since November 2024, the US and UK have allowed Ukraine to use their missiles to hit targets within Russian territory.
How is Nato increasing its defences against Russia?
In January and February 2025, Nato forces will take part in a military exercise called Steadfast Dart 25, external in Bulgaria and Romania, led by a UK army division.
Troops will practise how to reinforce the alliance's eastern borders ahead of an enemy attack, external.
In 2023, Nato's commanders agreed detailed plans for countering possible Russian attacks anywhere in the Arctic, the North Atlantic, Eastern Europe or the Mediterranean Sea region.
It increased the number of troops in Europe on high alert from 40,000 to more than 300,000, and bolstered its defences on Russia's borders with eight battlegroups, external.
-BBC