DONETSK, Ukraine -- Pro-Russian militants seized police stations and other security facilities Saturday in the most populous part of eastern Ukraine, actions that the government in Kiev immediately denounced as Russian "aggression."
The attacks on the Police Headquarters in Donetsk and on a police station and a state security branch in Slovyansk about 50 miles away, along with reports of shootings in several other towns, suggested a coordinated campaign to destabilize the Donetsk region, a vitally important industrial and coal-mining area that borders Russia.
Six days earlier, pro-Russian activists seized the headquarters of the regional government, declared an independent People's Republic of Donetsk, and demanded a referendum on whether to secede from Ukraine and join Russia.
Ukraine's acting president, Oleksandr Turchynov, called an emergency meeting late Saturday of the country's national security council to discuss the escalating crisis in the mainly Russian-speaking region.
Fears that the government is losing control have been fueled by the militants' seizing of a large number of weapons during the past week. Some 300 automatic rifles were taken from the Donetsk offices of the state security service after it was briefly taken over by pro-Russian activists last weekend, and according to the Ukrainian Interior Ministry, 400 Makarov handguns and 20 automatic weapons were looted Saturday from the police station in Slovyansk, which had been seized.
"The goal of the takeover was the guns," the ministry said in a statement.
The demands of the pro-Russian activists in eastern Ukraine keep shifting between outright secession and greater autonomy within Ukraine. But calls for unity with Russia now seem to predominate, heightening concerns in the West that Moscow is orchestrating the disorder to create a pretext for an invasion. Russian troops have been massed for weeks near the Ukrainian border.
In Washington, the White House expressed concern Saturday at "the concerted campaign we see underway in eastern Ukraine today by pro-Russian separatists, apparently with support from Russia," and warned Moscow not to intervene in Ukraine.
In Moscow, Russia's Foreign Minister Sergey Lavrov warned that any use of force against the pro-Russian activists would undermine talks meant to resolve the crisis in Ukraine, involving Russia, Ukraine, the European Union and the United States and scheduled for April 17 in Geneva.
Unlike the armed men who, earlier this year as a prelude to Russia's annexation of the peninsula, seized Ukrainian government buildings and bases in Crimea -- and later turned out to be Russian soldiers -- the gunmen behind Saturday's attacks in Donetsk, the home region of the ousted Ukrainian president, Viktor Yanukovych, appeared to be Russian-speaking local residents and not professional Russian troops.
Even so, Arsen Avakov, the acting interior minister in the shaky new Ukrainian government that came to power after Mr. Yanukovych fled from Kiev on Feb. 21, immediately blamed Russia for the Donetsk attacks, citing the use of weapons that he said only the Russian military has. In a posting on his Facebook page, Mr. Avakov said the "Ukrainian government considers today's facts as a manifestation of external aggression by the Russian Federation."
Mr. Avakov said that troops from his ministry and the Ukrainian military were "implementing the operational response plan." He did not elaborate, and the loyalty of the Ukrainian security forces based in eastern regions is regarded as uncertain.
Donbass News, a local media organization, reported Saturday that the head of the regional branch of Ukraine's state security service, Valery Ivanov, had been fired by the authorities in Kiev. It gave no reason. Opponents of the pro-Russia activists in Donetsk have accused the region's police and security services of sympathizing with the secessionist militants.
Some of the men who stormed the Donetsk police building Saturday, according to witnesses, wore the uniforms of the Berkut, a riot police squad that clashed with pro-Europe demonstrators in Kiev when Mr. Yanukovych was in power.