"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," Jens Stoltenberg said, as quoted by the BBC.
The head of the Western military alliance was speaking after former Russian spy Sergei Skripal, 66, and his 33-year-old daughter were targeted in a suspected nerve agent attack in Salisbury in early March.
They were found unconscious on a bench in the town in southern England on March 4 and are reported to be in intensive care, fighting for their lives in a local hospital.
The UK believes Russia is behind the poisoning, although Moscow denies involvement.
The BBC quoted Stoltenberg as saying that NATO had "no reason to doubt the findings and assessments by the British government" which point to Russian responsibility.
NATO is undergoing its biggest strengthening of collective defence since the end of the Cold War, Stoltenberg also said, according to the BBC.
Meanwhile, The Telegraph newspaper has quoted a UK police officer as saying that 131 people could have potentially come into contact with the deadly nerve agent in Salisbury.
According to the BBC, however, none of these people “has shown any symptoms.”
'Strong political support' for UK
Stoltenberg was also quoted by the BBC as saying that the "UK is not alone" amid a dispute with Russia over the Salisbury attack and that NATO allies give "strong political support" to Britain.
Politicians including US President Donald Trump, French President Emmanuel Macron and German Chancellor Angela Merkel have all declared support for the British government.
Donald Tusk, the head of the European Council, the EU’s top political authority, has said he will put the issue on the agenda for a summit of European Union leaders in Brussels next week.
Poland ready to help
Polish President Andrzej Duda said on Wednesday that those responsible for the suspected nerve gas attack in Britain “should be identified and punished.”
In a statement for the PAP news agency, Duda said that “the use of chemical weapons on the territory of our strategic ally cannot be left unanswered.”
He also said that Poland would “work closely with the United Kingdom on this issue within the UN Security Council and as part of NATO, the European Union and the Organisation for the Prohibition of Chemical Weapons.”
A senior aide to the Polish prime minister said on Wednesday that Poland is ready to help the UK solve the case of the suspected poisoning.
A former agent with Russia’s FSB security service, Colonel Skripal arrived in Britain in 2010 as part of a prisoner exchange; prior to that he was jailed in Moscow for spying for Britain, according to The Telegraph.
The British newspaper has reported that intelligence agencies believe the nerve agent that poisoned Skripal was planted in his daughter’s suitcase.