President Volodymyr Zelenskiy has urged the Ukrainian military to remain on alert despite Russia's drawdown of its troops from the country's borders, saying they could return "at any moment."
Kyiv has been battling Moscow-backed separatists in eastern Ukraine since 2014, following Moscow's annexation of the Crimean Peninsula.
A Russian troop buildup in recent weeks near Ukraine's northern and eastern borders and in Crimea has raised concerns of a major escalation of the conflict in Kyiv and in the West.
But on April 23, Moscow announced that it had started withdrawing its armed forces.
"The fact the troops are withdrawing doesn't mean the army should not be ready for the possibility troops could return to our borders any moment," Zelenskiy said while visiting Ukrainian military positions near Crimea on April 27 .
In Moscow, Russian Defense Minister Sergei Shoigu said that the troop pullback had nothing to do with Western pressure, adding that Moscow will continue doing what is necessary to protect itself.
Shoigu also voiced concern about the presence of NATO forces near Russia.
"Some even warned us that our activities on our own territory will have consequences," Shoigu said on April 27. "I would like to emphasize that we don't see such warnings as acceptable and will do everything that is necessary to ensure the security of our borders."
Pentagon Press Secretary John Kirby said this week that Washington had registered movements of some Russian troops away from Ukraine's borders, but added that it was "too soon to tell" whether Russia was pulling back all forces.
A cease-fire that took hold in July has been unravelling recently, with clashes sharply increasing between Ukrainian forces and separatists.
Around 30 Ukrainian soldiers have been killed since the start of the year compared with 50 in all of last year, while the separatists have reported at least 20 military deaths.
On April 27, the Ukrainian Army reported one soldier killed and three others wounded after their vehicle hit a mine.