Ukraine's interim president says his government is "helpless" to quell a growing pro-Russian separatist movement in two eastern regions, and he says the situation has deteriorated to the point Kyiv can not control its own troops.

Oleksandr Turchynov's admission came Wednesday at a meeting with regional governors in Kyiv. He singled out mounting difficulties in the Donetsk and Luhansk regions, where he said some military units are "cooperating with terrorist organizations."

Turchynov said his government will instead shift its focus to maintaining control of the nearby Kharkiv and Odessa regions.

The mayor of Kharkiv, who has been credited with maintaining calm in Ukraine's second city, was shot in the back earlier this week while cycling, as separatists tighten their grip on large parts of the east.

Mr. Turchynov told regional governors that the threat of a Russian invasion is real, and that Ukraine's military remains on "full alert."

Hours before Mr. Turchynov spoke, local authorities in the east said gunmen had seized control of a city council building in Horlivka, a city of 290,000 residents north of the industrial hub of Donetsk. Separatists now control about a dozen cities in Ukraine's industrial east, including Donetsk, where rebels have set a referendum on secession for May 11.

A similar vote last month led to Russia's annexation of the Crimean peninsula.

Meanwhile, negotiations continue for the release of seven European observers taken hostage last week in the eastern city of Slovyansk.

The self-proclaimed pro-Russian mayor of that city said Tuesday he would be willing to swap the observers for pro-Russian activists held by Ukrainian authorities. On Wednesday, he told reporters the talks are hindered by "technical" matters. He did not elaborate.

In Washington Wednesday, the top Republican on the Senate Foreign Relations Committee, Bob Corker, called for deeper sanctions targeting Russia's energy and banking sectors. He said current sanctions aimed at forcing Moscow to ease tensions in Ukraine are not adequate.