Secretary general Jens Stoltenberg said that Nato would put in four additional multinational “battle groups” in the neighbouring countries of Bulgaria, Hungary, Romania, and Slovakia, but insisted that measures remain “preventive, proportionate, and non-escalatory”.
The summit was attended by heads of state including UK prime minister Boris Johnson, Canada's prime minister Justin Trudeau and US president Joe Biden.
The defensive alliance also urged China not to back Russia's war effort in Ukraine or help Vladimir Putin escape the sanctions imposed following the invasion. Mr Stoltenberg called on Beijing to use its "significant influence" on Russia to press for an immediate peaceful resolution to the crisis.
Mr Stoltenberg, whose mandate in the role of of Secretary-General was yesterday extended for an extra year to help steer the 30-nation military organisation through the crisis, said: "Today we agreed to do more including cyber-security assistance and equipment to help Ukraine protect against biological, chemical, radiological and nuclear threats."
This could include detection equipment, protection and medical supplies, as well as training for decontamination and "crisis management".
Mr Stoltenberg said Nato's chemical, biological, radiological and nuclear defence elements had been activated and "we are taking measures both to support Ukraine and to defend ourselves".
In a statement, Nato described the situation as the “gravest threat to Euro-Atlantic security in decades”.
It added: “Russia’s unprovoked war against Ukraine represents a fundamental challenge to the values and norms that have brought security and prosperity to all on the European continent. President Putin’s choice to attack Ukraine is a strategic mistake, with grave consequences also for Russia and the Russian people. We remain united and resolute in our determination to oppose Russia’s aggression, aid the government and the people of Ukraine, and defend the security of all Allies.
Asked whether the move to increase protection against chemical and nuclear warfare was based on credible intelligence, Mr Stoltenberg said: "We are concerned, partly because we see the rhetoric and we see that Russia is trying to create some kind of pretext - accusing Ukraine, the United States, Nato allies - for preparing to use chemical and biological weapons."
He said any use of chemical weapons "will totally change the nature of the conflict, it will be a blatant violation of international law and it will have widespread consequences".
He added that Russia had used chemical weapons before - including in Salisbury - and had supported the Assad regime in Syria.
This comes as the Association for Ukrainians in Great Britain (AUGB) called on the West to continue to support Ukraine as the war enters its second month, warning that any weakening of the West's military support for Ukraine would “fuel his ambitions to recreate a Soviet sphere of influence”.
The AUGB said: “The invasion of Ukraine has shown the world that Ukrainians are determined to fight for their freedom with an inspiring strength and resilience that has stopped the Russian invading forces in their tracks. This has come at a huge cost to the civilian population. Russia’s total disregard for human life has created a humanitarian crisis, with thousands still trapped in bombed-out cities or no access to water or food.
"We call on the UK government and its Nato partners to continue to support Ukraine in every way they can. The highest priority is to stop the war, by providing Ukraine with the weaponry and equipment it needs to fight back. This must include aircraft and air defences, as well as protective equipment for Ukraine's Territorial Defence Forces. Western partners must not believe that Russia is serious about peace while Russia continues to kill innocent civilians. President Putin understands only strength and any weakening of the West's military support for Ukraine will simply fuel his ambitions to recreate a Soviet sphere of influence.”
Meanwhile, the World Health Organisation (WHO) warned that one month of war has had a devastating impact on Ukraine’s health system, severely restricted access to services, and triggered an urgent need to treat trauma injuries and chronic conditions.
In an in-depth report on the first 28 days of war, WHO said close to seven million people in the country are internally displaced, and the number of people that have fled to neighbouring countries is close to four million, WHO said – accounting for one in four Ukrainians. Separate figures published by UNICEF found that half of all Ukrainian children have been displaced since the war began.
The health body warned that the problems had exacerbated the conditions of those suffering from noncommunicable diseases, with figures from the International Organisation for Migration (IOM), indicating that one in three of those who have been internally displaced suffering from a chronic condition.
WHO said that a number of hospitals have been repurposed to care for the wounded – which it said comes at the cost of essential services and primary health care, with treatment of chronic conditions such as cancer having “almost stopped”.
WHO said it had verified 64 incidents of attacks on health care in 25 days to 21 March, causing 15 deaths and 37 injuries.
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“Attacks on health care are a violation of international humanitarian law, but a disturbingly common tactic of war – they destroy critical infrastructure, but worse, they destroy hope,” said Dr Jarno Habicht, WHO representative in Ukraine. “They deprive already vulnerable people of care that is often the difference between life and death. Health care is not – and should never be – a target.”
WHO added that approximately half of Ukraine’s pharmacies are thought to be closed, while Covid vaccination and routine immunisation has also come to a halt. Many health workers are displaced themselves or unable to work.