Kyiv has accused Russia of massing troops on its northern and eastern borders and in the Crimean peninsula annexed by Moscow in 2014.
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The European Union has expressed "great concern" over the movement of Russian military forces near the border with Ukraine.
Josep Borrell, the bloc's foreign affairs chief, also expressed "unwavering support" for Kyiv after a phone conversation with Ukraine's foreign minister Dmytro Kuleba.
"We are following with severe concern the Russian military activity surrounding Ukraine," Borrell tweeted on Sunday.
"Unwavering EU support for Ukraine's sovereignty and territorial integrity," he added.
Borrell said he would hold talks with Kuleba and the EU's 27 foreign ministers at an upcoming meeting in April. Other western leaders, including incoming US President Joe Biden, have said they stand by Ukraine.
Kyiv has accused Russia of massing troops on its northern and eastern borders and in the Crimean peninsula annexed by Moscow in 2014.
The Kremlin did not deny the recent military movements but insisted it was "not threatening" anyone.
The conflict in eastern Ukraine has killed more than 13,000 people since 2014, according to the United Nations.