Russia has positioned some 150,000 troops close to Ukraine's borders, according to US president Joe Biden.
They include about 30,000 troops taking part in military exercises in Belarus, and are equipped with everything from tanks and artillery to air power and naval support.
US officials say Russia has the troops in place to invade Ukraine "at any time".
But Russia denies it is planning an attack, and on Tuesday Moscow announced that some troops positioned near Ukraine were being pulled back.
Multiple rocket launchers being fire during the Allied Resolve 2022 joint military drills by Belarusian and Russian troops
Russia's defence ministry said, whilst large-scale drills were continuing, some troops from the southern and western military districts had completed their exercises and were returning to their permanent bases.
But on Wednesday, Nato said it had not seen evidence of any de-escalation on the ground.
Its secretary general Jens Stoltenberg said: "On the contrary, it appears that Russia continues the military build-up."
Most current estimates have put the number of Russian troops deployed around Ukraine at between 130,000 and 150,000.
Last week, western officials said the number had reached 130,000, up from their previous estimate of 100,000.
The US figure of 150,000 came in a speech by Mr Biden on Tuesday.
On Wednesday, US officials said that Russian troop numbers had increased by 7,000 in recent days.
The UK defence secretary Ben Wallace said that 60% of Russia's land forces were near Ukraine's borders with Russia and Belarus.
He added that he had seen evidence of troops moving from assembly areas to what he called "more aggressive launch areas".
Speaking to the BBC on Thursday, Russia's deputy ambassador to the UN, Dmitri Polanski said: "All this is happening in the heads of our western colleagues."
"I don't trust figures from British and American intelligence," he said.
Analysts in Ukraine have suggested a similar figure of 147,000, including air and naval personnel.
In addition to the regular Russian troops, there are thought to be about 15,000 Russian separatists in Ukraine's Luhansk and Donetsk regions. Ukraine believes the figure is higher.
Reports suggest that key support units necessary for an invasion have moved into position alongside combat troops in recent days.
They are thought to include tank repair workshops, mud clearance equipment and field hospitals with blood supplies in some areas.
On Wednesday, Mr Wallace highlighted a "continued build-up of field hospitals."
The prime minister Boris Johnson has said that the presence of field hospitals "can only be construed as a preparation for invasion".
Units arriving from across Russia have been added to about 35,000 military personnel who are permanently stationed close to Ukraine's border.
Some units have travelled almost 4,000 miles from the Russian Far East.
Much of the heavy armour has been brought in by rail, some passing through Kursk, approximately 80 miles (130km) from the Ukraine border.
Other vehicles have arrived by road, via Karachev in the Bryansk region.
Many experts believe that a full-scale invasion, followed by an occupation of most or all of Ukraine, would require many more troops than Russia has currently assembled.
Armoured vehicles can be identified by their shapes, and tyre tracks or mud indicate vehicles on the move.
Images of the Yelnya training ground appear to show a large pull-out of vehicles between January and February.
Some analysts believe they headed to areas closer to Ukraine's border.
Satellites have also detected a sharp rise in activity in Crimea where, according to one assessment, an additional 10,000 troops arrived in late January and early February.
It's believed they include infantry and airborne forces, and that some units have been put on the highest level of readiness.
The country's leader, Alexander Lukashenko, supports Russian President Vladimir Putin.
The Ukrainian capital, Kyiv, is less than 100 miles (150km) from the Belarus border, and western observers say the exercise, Allied Resolve, could provide an opportunity to rehearse a mission against Ukraine.
Mr Stoltenberg said that 30,000 Russian combat troops had arrived in Belarus, in Moscow's biggest military deployment to the country since the end of the Cold War.
Satellite images appear to show Russian Iskander short-range ballistic missile launchers near Yelsk in Belarus, less than 45 miles (72km) from the Ukraine border.
Russian deployments to Belarus include air defences, munitions and medical support.
Mr Stoltenberg said that Russian Speznaz special operations forces were also on the ground.
S-400 air defence system, sent by Russia, are seen at the Brestsky training ground in Belarus
Advanced Su-25 ground attack planes have been pictured by satellite at Luninets airfield.
The build-up at sea
Russia is holding worldwide naval drills, from the Atlantic to the Pacific, running through February, involving about:
140 ships and support vessels
60 aircraft
10,000 personnel
The Admiral Essen frigate leaves Sevastopol to take part in exercises on 25 January
Six Russian Navy vessels which passed through the English Channel in January are now in the Black Sea.
They are capable of landing main battle tanks, personnel and armoured vehicles.
On Saturday, Russia's Black Sea fleet said more than 30 ships had started training exercises near Crimea.
The Pyotr Morgunov sails through the Bosphorus Strait en route to the Black Sea on 9 February
On Monday, a senior Russian military official said that Russia was ready to open fire on foreign ships and submarines that illegally enter its territorial waters, according to the Interfax news agency.
Moscow has issued coastal warnings citing missile and gunnery firing exercises in the Black Sea.
Some analysts say an amphibious landing by Russian troops would be extremely difficult and the naval forces may be a "feint" to draw Ukrainian ground forces away from more likely routes of attack on land.
