The conflict between pro-Russian gunmen and Ukraine’s authorities turned bloody on Sunday, with one security service officer killed and reports of people wounded on both sides, as the struggle for the country’s east escalated one week after separatists began systematically occupying government buildings.

Gunfire — the first reported ­between authorities and pro-Russian separatists in the east — erupted as Ukraine declared that it would deploy its armed forces in a “large-scale anti-terrorist operation” against the burgeoning revolt in the Donetsk region.

“The blood of Ukrainian heroes has been shed in a war which the Russian Federation is waging against Ukraine,” President Oleksandr Turchynov said in an address to the nation Sunday evening. “The aggressor has not stopped and is continuing to sow disorder in the east of the country.”

The armed assaults on government buildings in the eastern region, close to the Russian border, have alarmed not only leaders in Kiev — who accuse Moscow of a coordinated campaign of aggression against Ukraine — but also those in the West. The attacks, officials said, were reminiscent of the shadowy invasion of the Crimean Peninsula, which resulted in its annexation by Russia last month.

“Well, it has all the telltale signs of what we saw in Crimea,” Samantha Power, the U.S. ambassador to the United Nations, said Sunday on ABC’s “This Week With George Stephanopoulos.” “It’s professional, it’s coordinated, there’s nothing grass-roots-seeming about it.”

If the attacks continued, she warned, the United States would intensify its sanctions against Russia. As for the Kiev government, it lost Crimea without firing a shot and has vowed not to repeat the mistake in eastern Ukraine.

The West has been cautioning Ukraine against starting a shooting war with the separatists for fear that it would offer Russia, which has thousands of troops gathered across the border, a pretext for invasion. Ukrainian Interior Minister Arsen Avakov said, however, that he had no alternative but to begin an “anti-terrorist” campaign Sunday after days of urging the separatists to go home peacefully.

Russia’s Foreign Ministry issued a statement Sunday night calling Ukraine’s actions “criminal” and adding that “it is now the West’s responsibility to prevent civil war in Ukraine.”

At an emergency meeting of the U.N. Security Council on Sunday night, Russian Ambassador Vitaly Churkin warned of “extremely significant consequences” if Ukraine’s government follows through with orders to use military force.

“In just a few hours’ time, things might take an irreversible turn for the worse,” he said. He also indicated that Russia had agreed only “in principle” to attend a meeting on the crisis scheduled to take place Thursday among Secretary of State John F. Kerry and counterparts from Russia, Ukraine and the European Union.

“What do you think?” Churkin said. “Tomorrow, there’s going to be the use of armed force and hostilities . . . and we’re going to sign off on that meeting? That is going to be fundamentally undermined if military operations are commenced in the southeastern region of Ukraine.”

Power responded sharply, saying, “It is not the United States that has escalated this situation. It is the Russian Federation.”

“While we would like to place our faith in talks,” she added, “it is hard to reconcile the behavior of the Russian Federation, the propaganda of the Russian Federation, the military actions of the Russian Federation . . . it is hard to reconcile those acts with this appeal for diplomacy and de-escalation, an appeal which we wish were, in fact, sincere.”

Although Russia appeared to wish that the “Internet did not exist,” Power said, the world could see videos clearly indicating that what Churkin called “demonstrations and protests” were, in fact, organized actions by well-armed and organized paramilitary forces. She repeated international calls for Russia to pull back its tens of thousands of troops massed on the Ukrainian border.

The meeting was the 10th emergency council session on the Ukraine crisis, this time called by Russia. As it began, Churkin condemned “grotesque Russia-phobia” on the part of elements of the Kiev government for provoking a response from Russian-speaking minorities in eastern Ukraine.

Power, along with representatives from Britain and France, ridiculed Churkin’s description of the situation, and they criticized what they called intensive Russian propaganda and media control designed to isolate and control the population of eastern Ukraine.

But while Russia stood alone in its version of events on the ground, the council offered no solution other than to call for calm and dialogue.

In Washington, President Obama’s national security team discussed whether to move forward with additional sanctions against Russia. Although the administration has frozen assets and banned visas of individual Russians said to be “cronies” of the Russian president, Obama indicated that more serious measures would be implemented if Russian troops entered eastern Ukraine, including sanctions against economic sectors such as energy and mining.

The question now facing the administration is how to respond to alleged Russian destabilization that falls short of outright invasion.

A deadline from Kiev

Turchynov gave the separatists a deadline of 9 a.m. local time Monday to vacate the buildings and leave under an amnesty. Last week, they were given a deadline of Friday to do the same. The offer was ignored.

On Sunday night, there was little evidence that Russian supporters had any inclination to retreat. Last Monday, they overran the Donetsk regional administration building and have held it ever since. On Saturday, they took the Donetsk regional police headquarters, while men in camouflage overwhelmed the police department in Slavyansk, a town 55 miles from the city of Donetsk. By Sunday, they had stormed other towns in the region.

Turchynov said that a captain in Ukraine’s security service was fatally shot Sunday in a fight outside Slavyansk and that two colonels were wounded. Four residents were wounded, other reports said.

The annexation of Crimea has been wildly popular in Russia. Russian flags hang from many balconies in Moscow, and President Vladimir Putin’s approval ratings are higher than ever. Even so, there are objections in some quarters to the invasion and the Russian news media’s coverage, which has portrayed Ukraine’s Russian speakers as under threat and in need of protection. Moscow considers the Kiev government illegitimate, and the Russian news media routinely call it a “fascist junta” in the pay of the United States.

About 10,000 people in Moscow rallied Sunday to protest such coverage and a wider crackdown on independent media. Some carried Ukrainian flags. Others had posters in support of Ukrainians. They were far fewer than the tens of thousands who marched along the same route March 15 to protest Russian intervention in Ukraine.

Systematic assaults

Trouble roiled one town after another in eastern Ukraine over the weekend. Mariupol, a city of nearly 500,000 in the Donetsk region, was beset Sunday by several hundred separatists who occupied the city council and raised Russian and “Donetsk Republic” flags, recently introduced by separatists. Local news reports said the attackers called themselves “the people’s army of Donbass,” a reference to the region around ­Donetsk.

The city council in Makeyevka, about 16 miles east of Donetsk, also was seized, local reporters said, and about 1,000 demonstrators rallied outside wearing black-and-orange St. George ribbons, which represent the Soviet victory in World War II and have been adopted as an emblem of pro-Russian sentiment.

The police station, city council and prosecutor’s office in the small town of Yenakiev, 40 miles east of Donetsk, were seized Sunday, according to Ukrainian newspaper reports. Former Ukrainian president Viktor Yanukovych, accused of helping Moscow stir up unrest, comes from the town.

Yanukovych fled to Russia in February after three months of protests against his government, set off by his refusal to sign an agreement for closer cooperation with the European Union. Instead, he wanted to pursue deeper economic ties with Russia, an inclination more popular in the Russian-speaking east than in the rest of Ukraine.

In Kramatorsk, a city of 200,000 about 60 miles north of Donetsk, the police headquarters was captured Saturday night by armed men who were shooting at the building and in the air. The city council building also was reported to be in separatist control.

Police and residents of the small town of Krasniy Liman reportedly fought off an attempt by separatists to seize offices there.

In the Kharkiv region north of Donetsk, 50 people were injured when separatists attacked a pro-Ukrainian rally with sticks and stun grenades, local police reported. No violence was reported in Luhansk, in the easternmost part of Ukraine.

The unrest has been accompanied by the presence of Russian troops along the border. NATO and U.S. officials estimate that 40,000 Russian troops are stationed there. Moscow has denied consistently that the troops pose a threat.

Turchynov’s announcement that the army would be used to tamp down the revolt comes amid reports of significant defections from the ranks of local police to the pro-Russian side. The army is also seen as a more appropriate force to use against a potential invasion.

The Ukrainian army, however, is poorly equipped, and Sen. John McCain (R-Ariz.) said Sunday that it was time for the United States to do something about it.

“We ought to at least, for God’s sake, give them some light weapons with which to defend themselves,” McCain said on CBS’s “Face the Nation.”

Karen DeYoung in Washington contributed to this report.