Russia has continued to build up its forces in annexed Crimea and near Ukraine in recent weeks, satellite images captured by a private US company show.
The latest images from Maxar Technologies in the US, could not be independently verified, Reuters reported.
More than 10,000 Russian troops have been returning to their permanent bases after month-long drills near Ukraine, Interfax news agency reported on Saturday, citing the Russian military.
The Kremlin had said on Friday that it reserves the right to move its own forces on Russian territory as it sees fit and that Western countries were carrying out provocative military manoeuvres near its borders.
US, European and Ukrainian leaders have accused Russia of building up troops again near Ukraine's border since October, after an earlier brief build-up in April, when Maxar also released images.
US President Joe Biden and other leaders say Moscow appears to be weighing an attack on Ukraine as soon as next month, something Moscow has repeatedly denied.
The images released late on Thursday show a base in Crimea, which Russia annexed from Ukraine in 2014, packed with hundreds of armoured vehicles and tanks as of December 13.
A Maxar satellite image of the same base in October showed the base was half empty.
Maxar said a new brigade-level unit, comprising several hundred armoured vehicles that include infantry fighting vehicles, tanks, self-propelled artillery and air-defence equipment, had arrived at the Russian garrison.
“Over the past month, our high-resolution satellite imagery has observed a number of new Russian deployments in Crimea as well as in several training areas in western Russia along the periphery of Ukraine border,” Maxar said.
It cited increased activity at three sites in Crimea and at five sites in western Russia.
Russian President Vladimir Putin said on Thursday that Moscow wanted to avoid conflict, but needed an “immediate” response from the US and its allies to its demands for security guarantees. Moscow has said it expects talks with US officials on the subject to start in January in Geneva.
On Friday, Mr Putin said Russia had conducted a test launch of a hypersonic missile. The test follows earlier ones this month, in October, and in July as part of what he said was a new generation of unrivalled arms systems.
When asked on Friday about the build-up of Russian troops near Ukraine, Kremlin spokesman Dmitry Peskov said Moscow was acting to defend its security.
“Russia is moving its own troops around on its own territory against the backdrop of highly unfriendly actions by our opponents in Nato, the United States and various European countries who are carrying out highly unambiguous manoeuvres near our borders,” Mr Peskov said.
“This forces us to take certain measures to guarantee our own security.”
Russia also accused the US and the EU of using the planned Nord Stream 2 pipeline as a bargaining chip. The West has threatened to block the pipeline, which is awaiting regulatory approval in Germany, if Russia invades Ukraine.
Mr Biden has threatened strong economic and other measures in retaliation for any invasion, building on sanctions imposed over Moscow's 2014 annexation of Crimea and backing for an continuing separatist rebellion by pro-Russian forces in eastern Ukraine. A US official has said new retaliatory measures could include tough export controls.
Russia says its wants Nato to halt its eastward expansion and is seeking guarantees that the Western military alliance will not deploy certain offensive weapons to Ukraine and other neighbouring countries.
Other Maxar images showed a build-up at the Soloti staging ground in Russia close to the Ukrainian border, with photos shot at the start of December showing a larger concentration of military hardware than in September.
Pictures also showed continuing build-ups at Yelnya, a Russian town about 260 kilometres north of the Ukrainian border, and at the Pogonovo training ground near the southern Russian city of Voronezh.