Russia will attack Ukraine unless Nato gives cast-iron guarantees that Kyiv will never be allowed to join the alliance, a Kremlin-linked foreign policy expert has said.

The suggestion by Fyodor Lukyanov is the clearest explanation yet as to why Russia has been amassing troops near the Ukrainian border, leading to a rise in East-West tensions and fears of an imminent invasion.

Mr Lukyanov – chairman of the board of the Russian Foreign Affairs Council, which advises the Kremlin – made it clear in an article published on Wednesday that Moscow would be seeking more than just verbal reassurances from Nato.

“This recent round of escalation in Eastern Europe showed that the old principles of security on the continent are no longer working,” he wrote.

He warned of a “new conflict” if Nato expanded further east.

“Russia will have to change the system and draw new ‘red lines,” he said, mentioning a post-war deal between the Soviet Union and Finland, under which Moscow recognised Finland’s independence in return for Helsinki’s neutrality in the Cold War. 

He added that the “gambit that led to the 2008 war between Russia and Georgia”, when Moscow invaded after claiming to have been provoked, “could well be replicated” in Ukraine.

Nato ‘is ready’ 

Vladimir Putin, the Russian president, has described the troop movement near the border as a deterrent against a Ukrainian government offensive in areas of the country held by Moscow-backed separatists.

Conflict Intelligence Team, a respected collective of Russian investigative journalists, put out a report on Wednesday based on open-sourced data that showed the Russian military moving tanks to new locations close to the Ukrainian border.

“We agree with estimates by (foreign) intelligence that Russia would be ready for a possible operation against Ukraine’s government-controlled territories no earlier than the beginning of next year,” it said.

In Brussels, Brigadier General Simon Doran, of the US Marine Corps, said of Nato’s preparation for a potential Russian invasion: “We exist to be ready at all times.”  

He told The Telegraph: “Hopefully, we’re not only deterring potential adversaries, we’re also reassuring all of our partners and allies that if called upon, we will be here.

“We are absolutely ready to combat any aggression from anybody globally.”

Ukraine this week held military exercises in the region bordering Russian-occupied Crimea to test the latest additions to its arsenal, including Turkish-made combat drones and American Javelin anti-tank missiles.

Lt Gen Valery Zaluzhny, the commander-in-chief of the Ukrainian armed forces, thanked Ukraine’s international allies for providing state-of-the-art weapons to fend off a possible Russian invasion. 

Oleksiy Reznikov, Ukraine’s defence minister, added that he had been briefed about Russia’s military movements near his country’s border in recent meetings with his British and US counterparts.

“We’re aware of the risks,” he said, adding that he had received assurances that Ukraine “won’t be left alone.”

Commenting on the reports of the Javelin missiles tested in Ukraine, Dmitry Peskov, the Kremlin’s spokesman, said that “the situation in the east remains very, very worrying.”

Russia held snap military drills the same day in the Black Sea near Crimea, involving three warships and ten fighter jets, the Russian military said.