Ukraine is deploying troops in a "large-scale anti-terrorist operation" to resist attacks by armed pro-Russian forces, Ukraine's President Oleksandr Turchynov said on Sunday in a televised address.
The previous president, who fled to Russia after being ousted earlier this year, accused the CIA of being behind the decision.
Turchynov said the Ukrainian Security Council decided to use the army because "we're not going to allow Russia to repeat the Crimean scenario in Ukraine's east." He pledged amnesty to anyone who lays down arms by Monday morning.
Speaking hours later on Russian state television, Viktor Yanukovych claimed that CIA director John Brennan had met with Ukraine's new leadership and "in fact sanctioned the use of weapons and provoked bloodshed."
The CIA flatly denied the accusation that Brennan was pulling the strings in Ukraine. CIA spokesman Dean Boyd said that while the agency doesn't comment on Brennan's travel itinerary, the "claim that director Brennan encouraged Ukrainian authorities to conduct tactical operations inside Ukraine is completely false."
Yanukovych was ousted in February following months of protests in Kiev, the capital, that were ignited by his decision to back away from closer relations with the European Union and turn toward Russia. He fled to Russia, saying he feared for his life.
Ukraine now has "one foot into a civil war," Yanukovych said Sunday. He was flanked by his former prosecutor general and interior minister, the two associates most despised by the Kiev protesters.
Ukrainian special forces exchanged gunfire with a pro-Russia militia outside an eastern city on Sunday morning, with one security officer killed and five others wounded. It was the first reported gunbattle in eastern Ukraine, where armed pro-Russian men have seized a number of government buildings in recent days.
Interior Minister Arsen Avakov has described such attacks as "Russian aggression." He said in a Facebook post Sunday that special forces of up to 12,000 people will be drawn from volunteers who will be tasked with resisting attacks from pro-Russian forces in their local areas.
Russia's Foreign Ministry was quick to dismiss Turchynov's decree as "criminal" and accused Ukrainian officials of using radical neo-Nazi forces.
The former Ukrainian interior minister, Vitaliy Zakharchenko, warned of "provocations" in the coming days to "discredit Russian armed forces."
Russia has tens of thousands of troops deployed along its border with Ukraine, which has raised fears of a military incursion, especially after the takeover of the Crimean Peninsula. The Russian government has denied any plans to intervene militarily in eastern Ukraine.
Unrest has spread to several municipalities in eastern Ukraine, including the major industrial city of Donetsk, which has a large Russian-speaking population and was the support base for Yanukovych. Ethnic Russians in Ukraine's east widely fear that the new pro-Western Ukrainian government will suppress them.
Pro-Russian demonstrators have demanded a referendum on autonomy and possible annexation by Russia, following the pattern set by the vote in Crimea last month.
In Sunday's gunbattle, Turchynov said a Security Service captain was killed and two colonels wounded outside Slovyansk, where the police station and the Security Service office were seized a day earlier.
An Associated Press reporter found a bullet-ridden SUV on the side of the road and a pool of blood on the passenger seat where the shooting was supposed to have taken place.
Vladimir Kolodchenko, a lawmaker from the area who witnessed the attack, said a car with four gunmen pulled up on the road in a wooden area outside Slovyansk and opened fire on Ukrainian soldiers who were standing beside their vehicles. Both attackers and the Ukrainian servicemen left soon after the shooting.
The regional administration in Donetsk issued a statement confirming one dead and saying nine were wounded. It did not identify them.
Ukraine's Foreign Ministry issued a statement late Sunday afternoon accusing "the Russian special service and saboteurs" of fomenting unrest and pledging to present "concrete evidence" of Russia's involvement at the Ukraine summit in Geneva on Thursday.
Ukrainian lawmaker Oleh Lyashko said Sunday afternoon that Ukrainian forces in Slovyansk had managed to take control of the city hall, the Security Service's branch and the police station. This could not be immediately verified.
Earlier in the day, the police station was surrounded by a reinforced line of barricades, but unlike on Saturday the men patrolling were largely unarmed. On the main road into the city, a checkpoint was guarded by armed camouflaged men.
In a phone call with Russian Foreign Minister Sergey Lavrov late Saturday, U.S. Secretary of State John Kerry "expressed strong concern" that the attacks "were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea," according the State Department.
The Russian Foreign Ministry denied Kerry's claims, while Lavrov blamed the crisis in Ukraine on the failure of the Kiev government "to take into account the legitimate needs and interests of the Russian and Russian-speaking population," the ministry said. Lavrov also warned that Russia may pull out of the Ukraine summit if Kiev uses force against "residents of the southeast who were driven to despair."
The UN Security Council has called an emergency meeting Sunday evening at Russia's request to discuss the growing crisis in Ukraine.
Two rival rallies in another regional capital in eastern Ukraine, Kharkiv, turned violent. At the end of both rallies, a group of pro-Russian protesters followed several pro-Ukrainian activists, beating them with bats and sticks, Interfax Ukraine reported. A video on Espresso TV showed one activist with blood on his head and hands waiting for paramedics on the steps of the underground passage. Several men and women came up to him and started kicking him.
An attack also was reported on a police station in the nearby city of Kramatorsk. A video from local news website Kramatorsk.info showed a group of camouflaged men armed with automatic weapons storming a police building. The news website also reported that supporters of the separatist Donetsk People's Republic occupied the administration building, built a barricade with tires around it and put a Russian flag nearby.
Regional news website OstroV said three key administrative buildings were seized in another city in the area, Enakiyeve. In Mariupol, a city south of Donetsk on the Azov Sea and just 50 kilometers (30 miles) away from the Russian border, the city hall was seized by armed masked men. Local news website 0629.com.ua said 1,000 protesters were building a barricade around it while unknown armed men raised the Russian flag over the building.