Nato has warned Belarus that it may face further fallout for diverting a passenger plane to arrest the dissident journalist Roman Protasevich and his partner.
Nato's Secretary General Jens Stoltenberg said it needed to be made clear to Belarus that "violating basic international norms and rules" would result in "costs" being imposed.
"I think the most important thing now is to make sure that those sanctions that are agreed are fully implemented. And I also know that other allies are looking into where they can step up further," he said in London, after meeting UK Prime Minister Boris Johnson.
"And I'm also sure that Nato leaders, when they meet, this will be an issue they will discuss as part of the response to the unacceptable behaviour of Belarus, but also as part of their response to a more assertive Russia," said Mr Stoltenberg, referring to the Nato meeting due later this month.
"Because this is part of the behaviour, we also see Russia and Belarus are working closely together."
Mr Johnson described Belarus's diversion of the plane as an "appalling, outrageous incident".
"I think Nato members will be wanting to stand together in protest against what happened and to call for the release of Roman Protasevich and indeed his girlfriend from captivity in Belarus," he said on Wednesday.
Mr Johnson also mentioned the "really outstanding" support the UK had received from Nato. He mentioned in particular the expulsion of Russian diplomats after the 2018 poisoning of double agent Sergei Skripal and his daughter Yulia in Salisbury, England, which has been blamed on Moscow.
"But what we want to do is make sure that we work together to defend, protect ourselves against cyber threats, against all the kinds of intimidation that some Nato members still feel there on Nato's eastern borders," said Mr Johnson.
"And we work together to protect against that."
Mr Protasevich, 26, was detained by Belarus on May 23 after his Ryanair flight was forced into an emergency landing in Minsk in an incident that caused global uproar.
Mr Stoltenberg's comments came after Nato recently launched a major training exercise in Europe.
On Monday, US B52 bombers flew over every Nato member to demonstrate the alliance's aerial prowess.
"Bomber missions demonstrate the credibility of our forces to address a global security environment that is more diverse and uncertain than at any other time in our history," said Gen Jeff Harrigian, the commander of the US air force in Europe and Africa.
Nato recently sent more than 9,000 troops, several warships and dozens of aircraft to the Atlantic and Europe as part of a separate exercise.
Nato said its Steadfast Defender 2021 exercise is designed to prepare its forces for any threat and ensure they work co-operatively.
While Nato said the exercise was not aimed at Russia specifically, it comes as tension remains high with Moscow on a number of fronts.
The alliance has stationed battlegroups in Estonia, Latvia, Lithuania and Poland – each of which borders Russia – in recent years, to establish its “forward presence” in the Black Sea region.
While Nato is at odds with Russia on a number of issues, Moscow’s deployment in April of thousands of soldiers to its border with Ukraine caused alarm.
"Nato is there to defend all our allies, and this exercise sends a message about our ability to transport a large number of troops, equipment across the Atlantic, across Europe and also to project maritime power," said Mr Stoltenberg last week, aboard the British aircraft carrier HMS Queen Elizabeth.
Russia, meanwhile, has announced the formation of 20 new units in the west of the country. Defence Minister Sergei Shoigu said the move was in response to Nato's frequent military drills, its deployment of warships and flights by US bombers near Russia territory.
He said such actions “destroy the international security system and force us to take the relevant countermeasures".
Mr Stoltenberg said Russia’s measures had led Nato to increase “the readiness” of its forces.
“Russia over the last years has invested heavily in new, modern military capabilities, from conventional to nuclear weapon systems”, and "has been willing to use military force against neighbours in Georgia, in Ukraine”, he said.
