The ministry said Russia Ambassador Oleg Vasnetsov had been told to appear at the ministry on Tuesday.

It said he would be asked to explain media reports that six polling stations would operate in the pro-Russian separatist region of Transdniestria.

It said this was at odds with an agreement to open a single station at Russia's embassy in Chisinau, capital of ex-Soviet Moldova, which lies between Ukraine and Romania.

Russians vote in a presidential election from March 15-17 in which incumbent Vladimir Putin is almost certain to win against three challengers, none of whom criticise him.

Moldova's pro-Russian president, Maia Sandu, has denounced Moscow's invasion of Ukraine, accuses the Kremlin of plotting a coup to unseat her and identifies Russia as the single biggest threat to her country's sovereignty.

Russia accuses Sandu of promoting anti-Russian sentiment and dismisses her bid to secure European Union membership as destructive for the country

Transdniestria split from Moldova as the Soviet Union was collapsing, fought a brief war against the newly independent state and has remained on the country's eastern border for more than three decades with virtually no violence or turmoil. Some 1,500 Russian "peacekeepers" remain in the territory.

Oazu Nantoi, a member of parliament from Sandu's PAS party, said the decision to open polling stations without permission was only the latest of numerous provocative Russian acts.

"This is an example of the provocative actions which Moscow uses to accuse Moldova of Russophobia, of violating the human rights of Russian citizens and of following in the steps of the scenario in Kyiv," Nantoi told Reuters.

Transdniestria has long been viewed as a possible flashpoint in and around Ukraine and the war has boosted tensions.

Also boosting tensions was the imposition of customs duties by central authorities in Chisinau on goods entering and leaving the enclave. Elected officials in Transdniestria appealed to Russia last month to provide them with "diplomatic assistance".

Moldova faces another regional threat from its southern region of Gagauzia, populated mainly by ethnic Turks. The region's leader, backed by an exiled businessman jailed in absentia for fraud, regularly promotes closer ties with Russia and last week met with Kremlin leader Putin.

(Reporting by Alexander Tanas; Editing by Michael Perry)

By Alexander Tanas