Ukraine sent an elite national guard unit to its southern port of Odessa, desperate to halt a spread of the fighting between government troops and a pro-Russian militia in the east that killed combatants on both sides Monday.

The government in Kiev intensified its attempts to bring both regions back under its control, but seemed particularly alarmed by the bloodshed in Odessa. It had been largely peaceful until Friday, when clashes killed 46 people, many of them in a government building that was set on fire.

The tensions in Ukraine also raised concerns in neighboring Moldova, another former Soviet republic, where the government said late Monday it had put its borders on alert. Moldova’s breakaway Trans-Dniester region, just northwest of Odessa and home to 1,500 Russian troops, is supported by Moscow, and many of its residents sympathize with the pro-Russian insurgency.

The loss of Odessa — in addition to a swath of industrial eastern Ukraine — would be catastrophic for the interim government in Kiev, leaving the country cut off from the Black Sea. Ukraine already lost a significant part of its coastline in March, when its Crimean Peninsula was annexed by Russia.

Compared with eastern Ukraine, Odessa is a wealthy city with an educated and ethnically diverse population of more than 1 million. Jews still make up 12 percent of the population of the city, which once had a large Jewish community.

“The people of Odessa are well-educated and understand perfectly well that Russia is sowing the seeds of civil war and destabilization in Ukraine,” said Vladimir Kureichik, a 52-year-old literature teacher who left Crimea after it became part of Russia.

In eastern Ukraine, gunfire and multiple explosions rang out in and around Slovyansk, a city of 125,000 in the Russian-speaking heartland that has become the focus of the armed insurgency against the government in Kiev.

Ukrainian Interior Minister Arsen Avakov said in a statement that government troops were battling about 800 pro-Russian forces, which were deploying large-caliber weapons and mortars. His ministry reported four officers killed and 30 wounded in the fighting.

The pro-Russian militia said at least eight people, both militiamen and local residents, were killed. A spokesman with the militia said that out of 10 people admitted to a hospital in Slovyansk with gunshot wounds, three later died. Five more were killed in fighting in the village of Semenivka.

The unrest in Odessa brought into question the loyalty of its police force. On Sunday, pro-Russian demonstrators stormed police headquarters and freed 67 people who had been detained in the rioting. Riot police simply stood by and did not interfere.

Presumably to prevent police from releasing more prisoners, the Interior Ministry said Monday that 42 others were being sent to another region for investigation.

The Interior Ministry also said it was sending an elite national guard unit from Kiev to re-establish control in Odessa, and the well-armed officers were seen on patrol.

On the outskirts of Kiev, checkpoints were set up Monday to control movement into the capital. Cars and buses with out-of-town license plates and other suspicious vehicles were stopped for inspection by police, working with the national guard and local volunteers.