Nato Secretary-General Jens Stoltenberg has expressed concern that Russia is continuing its military build-up around Ukraine, and that it has now deployed more troops and military equipment to Belarus than at any time in 30 years.
Meanwhile, more high-level diplomacy took place in Kyiv amid deep uncertainty about Russia's intentions.
Ukrainian President Volodymyr Zelenskyy held a three-hour meeting with Turkish President Recep Tayyip Erdogan in the Ukrainian capital.
French President Emmanuel Macron was due to hold phone talks with Mr Zelenskyy and Russian President Vladimir Putin.
Russia now has more than 100,000 troops stationed near Ukraine's northern and eastern borders, raising concern that Moscow might invade again, as it did in 2014, and destabilise the Ukrainian economy. Russian officials deny that an invasion is planned.
"Over the last days, we have seen a significant movement of Russian military forces into Belarus. This is the biggest Russian deployment there since the Cold War," Mr Stoltenberg told reporters at Nato headquarters in Brussels.
He said Russian troop numbers in Belarus are likely to climb to 30,000, with the backing of special forces, advanced fighter jets, Iskander short-range ballistic missiles and S-400 ground-to-air missile defence systems.
"So we speak about a wide range of modern military capabilities. All this will be combined with Russia's annual nuclear forces exercise, expected to take place this month," Mr Stoltenberg said.
Russian Defence Minister Sergei Shoigu was in Minsk to check on preparations for major Russia-Belarus war games scheduled for February 10 to February 20.
Mr Shoigu met with Belarusian President Alexander Lukashenko. Speaking about the drills, Mr Lukashenko said the goal was "to reinforce the border with Ukraine".
At the same time, Belarus' defence ministry accused Ukraine of violating the country's airspace with a drone last month.
The ministry summoned Ukraine's defence attache and handed him a note of protest over "frequent violations of the state border" with Belarus.
Kyiv rejected the allegation and accused Belarus of working with Russia to try to further unsettle Ukraine.
"We call on Minsk to refrain from playing along with Russia's destabilising activities," foreign ministry spokesman Oleg Nikolenko said on Twitter.
Ukraine's defence minister sought again to project calm, saying the probability of an invasion was "low", and he welcomed a change by US officials, who have stopped using the term "imminent" when describing the risk of a Russian attack.
Oleksii Reznikov said "the threat exists, the risks exist, but they have existed since 2014, ever since Russia has become an aggressor".
He said "there are no grounds for panic, fear, flight or the packing of bags". The minister put the number of Russian troops near Ukraine at 115,000.
Still, Mr Stoltenberg renewed his call for Russia to "de-escalate," and repeated warnings from the West that "any further Russian aggression would have severe consequences and carry a heavy price".
Nato has no intention of deploying troops to Ukraine should Russia invade, but it has begun to reinforce the defences of nearby member countries - notably Estonia, Latvia, Lithuania and Poland.
The 30-nation military alliance also plans to beef up its defences in the Black Sea region near Bulgaria and Romania.
Mr Stoltenberg also embraced President Joe Biden's decision on Wednesday to send 2,000 US-based troops to Poland and Germany and to shift 1,000 more from Germany to Romania, demonstrating to both allies and foes Washington's commitment to Nato's eastern flank.
"We are committed to finding a political solution to the crisis, but we have to be prepared for the worst," Mr Stoltenberg said, and he appreciated other recent offers of troops and equipment from several allies. Russia objects to the troop move and has described it as "destructive".
Mr Erdogan, a prominent Nato ally in the Black Sea region, is positioning himself as a possible mediator.
Speaking before departing for Kyiv, he reiterated Turkey's support for Ukraine's territorial integrity and said Ankara was ready to do what it can to reduce tensions.
"We are closely following the challenges that Ukraine is faced with as well as the tension in the region," he said.
"As a Black Sea nation, we invite all sides to exercise restraint and dialogue in order to bring peace to the region."
In Helsinki, Finnish leaders held talks with European Commission President Ursula von der Leyen about a letter that Russian Foreign Minister Sergey Lavrov sent to several countries on the "indivisibility of security" in Europe.
Mr Lavrov argues that the US and Nato misunderstand the concept - which essentially means that the security of one European country is linked to the security of them all - and he has demanded replies from countries that signed a key security document encompassing it to clarify the issue.
Finnish Prime Minister Sanna Marin said there was no "big news" in the letter but that it warranted a reply.
Ms Von der Leyen said the commission, the EU's executive branch, will coordinate a response, even though Mr Lavrov insisted that only countries and not organisations should answer.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules here