The Nato secretary general, Jens Stoltenberg, has cautioned Moscow against “any further provocation or aggressive actions” following warnings by US officials that Russia could be preparing to a launch a winter offensive in Ukraine. The Ukrainian president, Volodymyr Zelenskiy, said last week that Russia had amassed nearly 100,000 soldiers near Ukraine’s border, as Washington warned that Moscow may be “attempting to rehash” its 2014 invasion. “We have seen large and unusual concentrations of Russian forces close to Ukraine’s borders,” Stoltenberg said on Monday. “Nato remains vigilant … Any further provocation or aggressive actions by Russia would be of serious concern. We call on Russia to be transparent about its military activities.” Stoltenberg’s remarks came at a meeting with the Ukrainian foreign minister, Dmytro Kuleba, at Nato headquarters in Brussels, where the two discussed “the security situation in and around Ukraine”. “We have to be clear-eyed,” Stoltenberg said. “We need to be realistic about the challenges we face. And what we see is a significant, large Russian military buildup.” Satellite imagery and videos uploaded to social media have shown tanks, artillery and short-range ballistic missiles on the move in the border regions of Ukraine and Belarus since late last month. Some of that weaponry was moved to the region earlier this year during a large-scale buildup that set alarm bells ringing in the west. But new rail transports of military hardware have been spotted in southern Russia and from as far away as Russia’s Urals region, as well as on the Crimean peninsula annexed by Russia in 2014. While Stoltenberg said he did not want to speculate about the intentions of the Russian president, Vladimir Putin, he called the situation “dangerous” and said it would allow Russia to launch offensive operations at short notice. Analysts have noted that a previous Russian offensive into Ukraine against the city of Debaltseve in 2015 was also launched in winter, when the lower temperatures make it easier to transport armour and other heavy weaponry. British prime minister Boris Johnson argued on Monday that the west may soon have to stop relying on Russian oil and gas if it wants to stand against aggression towards Ukraine. In a warning to European countries that are heavily dependent on Russian gas in particular, the prime minister signalled he believed Putin could be serious about restricting supplies from pipelines that run through eastern Europe if the west signalled its intention to defend Ukraine. Speaking at the Lord Mayor’s banquet in London, Johnson said: “When our Polish friends asked for our help to deal with a contrived crisis on their border with Belarus, we were quick to respond. And we hope that our friends may recognise that a choice is shortly coming between mainlining ever more Russian hydrocarbons in giant new pipelines and sticking up for Ukraine and championing the cause of peace and stability, let me put it that way.” The US president, Joe Biden, dispatched the CIA director, Bill Burns, to Moscow this month to warn Putin that the US was watching the buildup closely, US media have reported. “I hope the whole world can now clearly see who really wants peace and who is concentrating nearly 100,000 soldiers at our border,” Zelenskiy said in a speech last week. Russia meanwhile has denied that it has any plans to launch offensive operations against Ukraine. Putin this weekend complained about Ukraine’s use of Turkish-made Bayraktar TB2 drone strikes in Donbass, saying that they violated a 2015 ceasefire agreement. And top Russian military officials have accused Nato of a buildup in Europe and in the Black Sea. The Russian military movements come as tensions increase over the migration crisis on Belarus’s border with the European Union. Belarus has warned that any conflict could draw in Russia as the two countries are allies and jointly defend the country’s borders with Nato. Moscow last week also sent nuclear-capable bombers and paratroopers on training missions to Belarus’s western borders with Poland and Lithuania as Moscow and Minsk held joint exercises. Lithuania’s foreign minister, Gabrielius Landsbergis, also suggested the Kremlin could be using the Belarus border crisis as a smokescreen to prepare an attack. “It is very likely that Ukraine could be attacked while we are dealing with the situation on the Polish, Lithuanian and Belarusian border,” he said. He added that the buildup was “actually the same distance from the border of Belarus and the same distance to the border of Ukraine, so I think that what Putin might be playing is that we don’t have an answer [are kept guessing] to who is under attack”.