NATO's chief has warned that the military alliance will defend every inch of its territory should Russia attack a member country, as he condemned Moscow for launching its brutal act of war on Ukraine.
Speaking after chairing an emergency meeting of NATO envoys, Secretary General Jens Stoltenberg said the 30-nation security alliance will continue to beef up its defences on its eastern flank near Ukraine and Russia.
He said US President Joe Biden and his NATO counterparts will hold an online summit on Friday.
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"Russia has attacked Ukraine. This is a brutal act of war. Our thoughts are with the brave people of Ukraine," Mr Stoltenberg told reporters.
"Peace in our continent has been shattered. We now have war in Europe, on a scale and of a type we thought belonged to history.
"NATO is the strongest alliance in history, and make no mistake we will defend every ally against any attack on every inch of NATO territory," he said at the organisation's Brussels headquarters.
"An attack on one ally will trigger a response from the whole alliance."
During the meeting, Bulgaria, the Czech Republic, Estonia, Latvia, Lithuania, Poland, Romania and Slovakia triggered urgent consultations under Article 4 of NATO's founding Washington Treaty.
These are launched when "the territorial integrity, political independence or security of any of the (NATO) parties is threatened".
Mr Stoltenberg said NATO has decided to activate emergency planning to allow commanders to move forces more quickly.
He spoke shortly after envoys to the transatlantic alliance agreed to further beef up its land, sea and air forces on its eastern flank near Ukraine and Russia.
"We have decided, in line with our defensive planning to protect all allies, to take additional steps to further strengthen deterrence and defence across the alliance," the ambassadors said in a statement.
"Our measures are, and remain, preventive, proportionate and non-escalatory."
Lithuania declared a state of emergency in a decree signed by President Gitanas Nauseda in response to Russia's attack.
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The Baltic country's parliament is expected to approve the measure in an extraordinary session on Thursday.
The measure, in effect until 10 March, allows for a more flexible use of state reserve funds and increased border protection, giving border guards greater authority to stop and search individuals and vehicles in border areas.
NATO member Lithuania borders Russia's Kaliningrad region to the south west, Belarus to the east, Latvia to the north and Poland to the south.
While some of NATO's 30 member countries are supplying arms, ammunition and other equipment to Ukraine, NATO as an organisation is not.
It will not launch any military action in support of Ukraine, which is a close partner but has no prospect of joining.
Estonia, Latvia and Lithuania, however, said in a joint statement: "We would need to urgently provide Ukrainian people with weapons, ammunition and any other kind of military support to defend itself as well as economic, financial and political assistance and support, humanitarian aid."
"The most effective response to Russia's aggression is unity," Estonian Prime Minister Kaja Kallas tweeted.
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"Russia’s widespread aggression is a threat to the entire world and to all NATO countries."
NATO began bolstering its defences in north-eastern Europe after Russia annexed Ukraine's Crimean Peninsula in 2014.
It has about 5,000 troops and equipment stationed there, but those forces have been beefed up with troops and equipment from several countries in recent months.
A first step now could be to activate the NATO Response Force (NRF), which can number up to 40,000 troops.
A quickly deployable land brigade that is part of the NRF – made up of around 5,000 troops and run by France alongside Germany, Poland, Portugal and Spain – is already on heightened alert.
Some NATO members have also sent troops, aircraft and warships to the Black Sea region, near allies Bulgaria, Romania and Turkey.
The UK's Ministry of Defence has announced a host of new British deployments in recent weeks.
The Pentagon has also put up to 8,500 US troops on heightened alert, so they will be prepared to deploy if needed to reassure other allies.