Pro-Russian activists have seized six Ukrainian armoured personnel carriers (APCs) along with their crews in Kramatorsk, in a major setback to the government's hopes of asserting authority in the east.
The Ukrainian Ministry of Defence said in a statement that the vehicles were blocked by local citizens, with members of a Russian terrorist sabotage group among them.
"The extremists captured vehicles and moved to Slavyans," the statement read. "The armoured vehicles are located near one of the administrative buildings in the centre of Slavyansk. Armed persons in military uniform are near them, [but] these persons are not from the Ukrainian Armed Forces."
The separatists also raised the Russian flag on the vehicles, which were allegedly taken from the Ukrainian troops who had driven into the town after taking control of a nearby airfield.
However, a spokesperson for the separatists was quoted by Reuters as saying that the Ukrainian soldiers surrendered their vehicles after talks.
The whereabouts of the Ukrainian servicemen is still unknown, according to the Ukrainian Defence Ministry.
Another column of 15 armoured troop carriers was reportedly surrounded and halted by a pro-Russian militia elsewhere in Kramatorsk, but left after the soldiers handed the firing pins from their rifles to a rebel commander.
After several days of delays, Ukraine launched an anti-terrorist operation on Tuesday, dispatching the 25th paratrooper division troops to the east to retake government buildings held by separatists who have proclaimed independence and demanded a referendum on joining Russia.
Meanwhile, representatives from Ukraine, Russia, the US and the EU are scheduled to meet today in Geneva, Switzerland to discuss the crisis in the former Soviet Republic.