Kiev warned Russia on Thursday against invading Ukraine, saying it would "pay dearly" for any such attempt, as the West fears Moscow might be plotting to send troops across the border.
Western countries have this month raised alarm over reported Russian military activity near Ukraine, with the United States saying it has "real concerns" over a new troop build-up on the border.
Speaking to reporters on Thursday, Ukrainian Foreign Minister Dmytro Kuleba said it was hard to guess what Russian President Vladimir Putin had in mind but Kiev was doing everything to deter the Kremlin.
"We are trying to make him understand that he will pay dearly for a new attack on Ukraine," Ukraine's top diplomat said.
"Moscow must clearly understand what political, economic, and human losses it will suffer in case of a new phase of aggression," Kuleba added. "So it's better not to do it."
On Wednesday, the U.S. Embassy in Ukraine released a security alert, warning the U.S. citizens of "unusual Russian military activity near Ukraine's borders and in occupied Crimea."
Moscow has dismissed growing Western claims that Russia may be preparing to invade Ukraine.
Putin on Wednesday told European Council President Charles Michel he was concerned by Kiev's "provocations" to inflame tensions in eastern Ukraine, the Kremlin said.
Kiev's army has been battling separatist fighters in two eastern regions bordering Russia since 2014, after Moscow annexed the Crimean peninsula from Ukraine.
The conflict has claimed more than 13,000 lives.
Ukraine and its Western allies accuse Russia of sending troops and arms across the border to support the separatists.
Moscow has denied the claims.