Ukraine's SBU security service says illegal military groups from Moldova's breakaway Transdniestria region worked with Russian groups to stir up unrest in the Ukrainian Black Sea city of Odessa, where dozens died in clashes on Friday.
It was not the first time largely Russian-speaking Transdniestria, a strip of Moldova bordering Ukraine and less than 100 km (62 miles) from Odessa, has been drawn into the crisis.
NATO's top military commander said in late March that the separatist region, which declared its independence in 1990, could be next in Russia's sights after its annexation of Crimea earlier in the month.
Russian President Vladimir Putin said last month the people of Transdniestria, who have voted in a referendum in favor of joining Russia one day, should have the right to decide their own fate, though he stressed the need for negotiation.
Moldova, one of Europe's poorest countries, is seeking closer ties with the European Union, which last month said Moldovans would no longer need visas to travel to most of the bloc.
Both Kiev and Moscow have alleged outside interference in clashes that have been concentrated in the mainly Russian-speaking south-east of Ukraine.
Russia's U.N. Ambassador Vitaly Churkin said English was heard over the radio as Ukrainian forces sought to dislodge pro-Russian rebels from the city of Slaviansk on Friday.
"English-speaking foreigners were noted," Churkin said.
Following is a brief profile of Transdniestria:
* A tiny sliver of land on the Dniester river, Transdniestria declared its independence from Moldova in September 1990. The region is dominated by Russian-speaking Slavs, who pressed for independence on fears Moldova's Romanian-speaking majority would one day become part of Romania to the south, restoring the status quo before the Soviet Union took control in 1940.
* The region fought a brief war with Moldova in 1992 that killed about 860 people from Transdniestria and 460 on the Moldovan side before Russian troops intervened. At least 1,200 Russian soldiers remain and guard some 20,000 tonnes of Soviet-era weaponry and ammunition.
* Subsequent referendums inside Transdniestria have produced big majorities for independence and for joining Russia one day. Mediation led by the Organization for Security and Cooperation in Europe has made little progress. Transdniestria has not been recognized by any sovereign state.
* The territory runs about 220 km (140 miles) down Moldova's eastern border with Ukraine and is 55 km (30 miles) at its widest point. Transdniestria has its own currency, police force and mandatory military conscription.
* Although it is home to just 550,000 people and occupies about one eighth of Moldovan territory, Transdniestria holds a significant share of Moldova's industry. Mostly Russian private investors own industrial companies including a steel plant and hydroelectric station. Some 50,000 Transdniestrians work in factories abroad, chiefly in Russia, and send home the cash that keeps the economy ticking over.
* The Sheriff conglomerate, owned by an ex-KGB official, reaches into huge chunks of Transdniestria's private sector including petrol stations, supermarkets, a football team and stadium and the biggest brandy retailer, Kvint. Western governments say Transdniestria has become a "black hole" for smuggling arms, cigarettes and other contraband, something the region's leaders deny.
(Reporting by Moscow and Kiev bureaux; Editing by Sophie Hares)