Nato Secretary General Jens Stoltenberg has warned US President-elect Donald Trump that "going it alone" is not an option for Europe or the US.
During his election campaign, Trump dismissed Nato, the western military alliance as obsolete. He also suggested that the US might not come to the assistance of a Nato ally under attack if they were not contributing enough financially to the military costs.
In a New York Times interview, Trump said: "I would be absolutely prepared to tell those countries, 'Congratulations, you will be defending yourself.'"
Stoltenberg called for unity. "Going it alone is not an option, either for Europe or for the US. We face the greatest challenges to our security in a generation.
This is no time to question the value of the partnership between Europe and the US," he said in the Observer.
"The only time Nato has invoked its self-defence clause, that an attack on one is an attack on all, was in support of the US after the 9/11 terrorist attacks. This was more than just a symbol.
"Nato went on to take charge of the operation in Afghanistan. Hundreds of thousands of European soldiers have served in Afghanistan since. And more than 1,000 have paid the ultimate price in an operation that is a direct response to an attack against the US."
The head of Nato raised concerns about Russia under Vladimir Putin's leadership, which he says is becoming more "assertive", with a Russian warship flotilla docking off the coast of Syria as well as military battalions on its western border.
"Nato battalions numbering thousands of troops cannot be compared with Russian divisions numbering tens of thousands just across the border. Our response is defensive and proportionate. But it sends a clear and unmistakable message: an attack against one will be met by a response from all," Stoltenberg said.
By Nato's count, total defence spending of all Nato members was calculated at around $900bn (£714.2bn) in 2015, with the US's share about $650bn or 72.2%.