"We've seen this before ... What does this mean? We don't know yet, too early to tell," Chairman of the Joint Chiefs of Staff General Mark Milley said.
Russia has left military units near its border with Ukraine after training exercises, with the number of troops in the area now totaling 90,000, according to the Ukrainian Defense Ministry.
Russian armed forces recently held a series of large-scale drills in the region, including airborne troops, the ministry said late on Tuesday.
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After the training, units of the 41st Army stayed behind, about 260 kilometers (160 miles) from the Ukrainian border,..
"It should be noted that the Russian Federation periodically resorted to the practice of transferring and accumulating military units in order to maintain tensions in the region and political pressure on neighboring states," the ministry said.
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Ukraine's Defense Ministry had earlier rejected recent media reports that Russia was massing troops near the two countries’ shared border, saying on Monday that it had not seen an increase in forces or weaponry.
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Kyiv's answer came after the U.S.-based Politico news outlet said commercial satellite photos taken on Monday "confirmed" reports that Russia was gathering troops and military equipment in the region.
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Satellite pictures supplied by U.S. space company Maxar Technologies, showed about 1,000 military vehicles near the Russian town of Yelnya, about 250 kilometers (150 miles) north of the Ukrainian border.
"Based on a review of recent satellite images, the equipment (which includes tanks, armored personnel carriers, self-propelled artillery, and associated support equipment) likely began arriving in the area in late September," Maxar told Reuters.
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The Kremlin denied the report and said that Russia has a military presence on its territory where necessary.
Relations between Kyiv and Moscow have worsened since 2014 when Russia annexed Ukraine's Crimean peninsula and war erupted between Ukrainian troops and Russian-backed separatist forces in eastern Ukraine.
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Ukraine says at least 14,000 people have died in the conflict.
Ukraine and its allies blame Russia for transferring troops and arms across its border to back the separatists fighting in its eastern Donetsk and Luhansk regions.
Moscow denies this and has repeatedly accused the U.S.-headed NATO Transatlantic Military Alliance of carrying out activities close to its borders.
The alliance says it is set to reinforce the security of member states close to Russia following Moscow's seizure of Crimea and its alleged involvement in the conflict in Ukraine, which is a NATO ally, but not a member.
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