The Russian Army will "respond" to any provocation by Ukrainian forces in Moldova's Moscow-backed separatist region of Transnistria, the Russian Foreign Ministry said on Friday.
"Russia's armed forces will respond accordingly if there is a provocation by the Kyiv regime," it said in a statement, accusing Ukraine of deploying a "large number" of soldiers and arms at the border.
Transnistria is a narrow region bordering Ukraine, which after a war separated from Moldova in 1992. Russian soldiers have been deployed there since.
Moldova, a poor country of 2.6 million people with a sizeable Russian minority, has taken a pro-Western turn in recent years, angering Moscow.
Since the launch of the offensive in Ukraine a year ago, the Kremlin has been accused of raising tensions in Transnistria to destabilise Ukraine and Moldova.
Russia's Defence Ministry on Thursday claimed that Ukraine had "stepped up preparations for the invasion" of Transnistria.
In Friday's statement, the foreign ministry said Russia's armed forces would "guarantee the defence of our compatriots, of Russia's peace-keeping contingent and of military storage facilities".
"Any action representing a threat to their security will be viewed in accordance with international law as an attack on the Russian Federation," it said.
The government of Moldova said Thursday that "state authorities do not confirm the information released this morning by the Russian Ministry of Defence".
"We call for calm and to obtain information from official and trusted sources of the Republic of Moldova," it said on its Telegram channel.
Earlier this month, Moldovan President Maia Sandu accused Russia of plotting to violently overthrow her government through saboteurs disguised as anti-government protesters, claims that Russia denied.