Secretary General Jens Stoltenberg said Nato was undergoing its biggest strengthening of collective defence since the end of the Cold War.
The UK believes Russia is behind the poisoning, although Moscow denies this.
Meanwhile, investigators say 131 people may have been exposed to the chemical used in the attack in Salisbury.
'UK is not alone'
Russian ex-spy Sergei Skripal, 66, and daughter Yulia Skripal, 33, remain critically ill in hospital, after they were found unconscious on a bench in the Wiltshire city on 4 March.
The UK government says they were poisoned with a nerve agent of a type developed by Russia called Novichok and PM Theresa May said she believed Moscow was 'culpable'.
Mrs May has said the UK will expel 23 Russian diplomats as part of a 'full an robust' response - prompting Russia's Foreign Minister Sergei Lavrov to say it will 'certainly' expel British diplomats in response.
Speaking to BBC Radio 4's Today programme on Friday, Mr Stoltenberg said Nato has 'no reason to doubt the findings and assessments by the British government' which suggest Russian responsibility.
He said the 'UK is not alone' and Nato allies give 'strong political support' to Britain, following a joint statement from the US, France and Germany backing Mrs May's government and a pledge of support from Australia.
Mr Stoltenberg said the incident was part of a 'pattern of reckless behaviour' from Russia following allegations of cyber attacks and election meddling in recent years.
'It is important that Russia gets a clear signal that it costs to behave the way they behave,' Mr Stoltenberg said.
'I'm absolutely certain that Russia has underestimated the resolve and unity of Nato allies when we have implemented different kinds of sanctions over the last years,' Mr Stoltenberg added.
But Kremlin spokesman Dmitry Peskov said Russia was not worried by international expressions of support for the UK and challenged Britain to 'provide some confirmation'.
He said: 'Sooner or later, the British will have to show some proof to those 'colleagues' who say they are with UK on this; sooner or later will have to stand up its accusations.'
It comes as Labour leader Jeremy Corbyn has written in the Guardian warning against 'hasty judgements', and not to 'rush ahead of the evidence'.
Foreign Secretary Boris Johnson has reassured Russians that the UK's dispute is specifically with 'Putin's Kremlin', claiming it is 'overwhelming likely it was his decision to direct the use of a nerve agent' in Britain.
Wiltshire Police said 131 people had been identified as potentially being exposed to the nerve agent - but none has shown any symptoms.
Salisbury District Hospital has also assessed 46 people who came forward expressing health concerns but they were not admitted.
In a letter to the Times, Salisbury NHS Trust emergency medical consultant Stephen Davies said only three people - the Skripals and Detective Sergeant Nick Bailey - had needed treatment.
Det Sgt Bailey remains in a serious but stable condition in hospital after being contaminated with the chemical.
Russia's ambassador to Britain Alexander Yakovenko claimed the UK had angled allegations against Russia to 'divert attention from Brexit'.
He criticised the lack of transparency and said: 'Nobody saw even the pictures of these people in a hospital, whether they are alive or maybe they are in good health. Nobody talked to the doctors.'
And Russia's Foreign Minister Sergei Lavrov said Russia had asked the UK to take action under the Chemical Weapons Convention.
He also responded to Defence Secretary Gavin Williamson - who previously said Russia should 'go away' and 'shut up' - saying: 'Maybe he lacks education.'
Some 220 police officers from 16 forces, 80 ambulance staff, 50 fire officers, 200 armed forces personnel and 250 specialist officers have so far been deployed as part of the investigation, Wiltshire Police said.
On Thursday, Mrs May visited Salisbury to speak to emergency service workers, public health experts and local business owners who are affected by the police cordons.
Wiltshire Council has announced measures to help affected businesses including free park and ride journeys and waived business rates.
Mr Skripal is a retired colonel in the Russian military intelligence service.
He was jailed by Moscow in 2006 for secretly working for Britain's MI6 but was later released and allowed to come to the UK.