Russia has officially begun the main phase of massive joint military exercises in Belarus, despite Western countries’ warnings that Russian forces massed near Ukraine could be used to launch an attack.

In the weeks leading up to the 10-day drills, Russia has poured an unprecedented amount of troops and equipment into Belarus, moving units nearly 6,000 miles from its far east and deploying tanks, long-range artillery, and advanced fighter jets.

The US and NATO countries have expressed concern that the exercises could be used as a cover for preparations for an attack on Ukraine, whose capital, Kyiv, is only about 200 miles away.

However, Russia and Belarus have insisted that the drills are merely defensive war games. The exercises, dubbed “Union Resolve 2022,” will “practice averting and repelling external aggression via a defensive operation, as well as combating terrorism and defending the interests” of Russia and Belarus, according to Russia’s defense ministry on Thursday.

The exercises are scheduled to end on February 20, and the Kremlin has stated that its troops will leave Belarus at that time.

Russia’s deployment of troops to Belarus is part of a larger military buildup that includes over 100,000 troops along Ukraine’s eastern border and in Crimea to the south. Independent military analysts have raised concerns about the drills, claiming that the Russian deployment is vastly larger than anything seen since the Cold War and includes units that would be used in a major invasion, such as advanced anti-air defenses and Iskander-M long-range missile brigades. Some of the Russian units have been spotted parked just a few dozen miles from Ukraine’s border, in areas not officially designated for the exercise.

The Belarus drills will take place at a time when Western countries and some analysts believe Russia will be ready to launch a major military operation against Ukraine.

NATO Secretary General Jens Stoltenberg warned on Thursday that “the number of Russian forces is increasing.” The warning time for a potential attack is decreasing.”

Ukrainian officials are far more skeptical, claiming that Russian forces in Belarus do not appear to be preparing to launch an offensive. Ukraine’s Defense Minister, Oleksiy Reznikov, said this week that he did not see Russia forming any strike groups in Belarus and that it had insufficient forces, estimating only a few thousand.

Ukraine’s government has stated that it believes Russia’s current buildup is primarily intended to put psychological pressure on Ukraine through the threat of an attack.

Russia will make a massive display of military power to the north, east, and south of Ukraine over the next 10 days, holding exercises on an unprecedented scale also outside Belarus. The period appears to be pivotal in determining whether the crisis escalates or if Russia’s buildup is a bluff.

The Belarus exercises will also coincide with large-scale Russian naval drills in the Black Sea, which prompted Ukraine to accuse Moscow on Thursday of launching a “unprecedented” naval blockade of Ukrainian ports. Six Russian warships, including amphibious landing ships, arrived in the Black Sea on Wednesday night to join an existing fleet. Ukraine has stated that it “strongly protests” the upcoming live-fire drills in the Black Sea and the Sea of Azov, which are expected to close off access to much of its coast in the Black Sea and the Sea of Azov.

If President Vladimir Putin is preparing to launch an attack, the Russian buildup appears to be nearing completion, according to some analysts, who say it has nearly completed its buildup of heavy equipment and is now entering a phase in which it will move up personnel to man it.

This does not imply that Russia will attack — US officials believe Putin has yet to make a decision — but rather that it will have the necessary forces in place within the next two weeks.

According to Janes, a defense think tank, there are at least 14 Russian battalion tactical groups in Belarus, with 8,000 to 14,000 troops. The United States has stated that it is assessing up to 30,000 could take part.