Oren Dorell

USA TODAY

One Ukrainian soldier was killed and seven others were wounded in combat with Russian-backed militants in eastern Ukraine, officials said Sunday, as escalating violence threatens a cease-fire that took hold in September.

Tensions were also high in the Black Sea port city of Odessa, where numerous armored vehicles belonging to the Ukrainian National Guard took up positions outside the Trade Union Building, site of clashes two years ago Monday that resulted in the deaths of dozens of pro-Russian demonstrators.

Sunday's fighting took place near the Svitlodarsk water reserve, between the rebel-held cities of Luhansk and Donetsk, in nearby Avdiivka and in Mariupol, Ukraine’s eastern-most city on the Black Sea, according to Ukraine’s military spokesman, Col. Oleksandr Motuzyanyk.

Fighting has been on the rise, but Orthodox Easter, celebrated Sunday, was supposed to mark the beginning of a return to the cease-fire, according to a statement by the Organization for Security and Cooperation in Europe (OSCE), a monitoring group, and German Foreign Minister Frank-Walter Steinmeier.

Violations of the cease-fire had been increasing weekly, both in number and intensity, but the holiday was a chance to prevent the situation from spiraling out of control again, Steinmeier said.

On Sunday, however, the Defense Intelligence unit of Ukraine’s Ministry of Defense reported that soldiers of Russia’s 10th Special Forces brigade arrived in the southern part of the Donetsk region, which is held by Russian-backed forces. Russia also sent 24 repaired tanks to the Luhansk region, plus 320 tons of ammunition and 1,000 tons of diesel fuel to other militant-held Ukrainian towns, according to a Ukrainian Defense Intelligence unit statement.

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The troop presence in Donetsk coincides with a visit to the region by Vladislav Surkov, assistant to Russian President Vladimir Putin, according to the Ukrainian Pravda newspaper. Surkov called on rebel forces to push Ukrainian military units from positions near Avdiivka and to do a better job of distributing Russian military and financial aid, Pravda reported, citing the Ukrainian military’s Defense Intelligence unit.

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In Odessa, 3,000 military and police officers, including 400 members of the Ukrainian National Guard, were deployed to maintain order during the May holidays, Unian reported.

The reinforcements were sent because of “the preparation by pro-Russian forces for violent actions and disorders in Odessa during the May holidays,” Aleksandr Tkachuk, chief of the Security Service of Ukraine, told TV 112, according to a translation by The Interpreter. Among the troops are special combat units referred to as “war surgeons,” whose job is to immediately react to possible threats, Tkachuk said.

The soldiers put on a display of force in front of the Trade Unions Building, site of violent clashes in 2014 between pro-Russian and pro-Ukrainian demonstrators that resulted in a fire and the deaths of 48 people, mostly on the pro-Russian side.

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Victoria Nuland, the U.S. assistant secretary of State for European and Eurasian affairs, on Friday urged Ukraine’s government to start jailing corrupt officials and to make the constitutional changes necessary to proceed with elections in rebel-held parts of the country, as stipulated in the cease-fire agreement reached in Minsk, Belarus.

“It's time to start locking up people who have ripped off the Ukrainian population for too long, and it is time to eradicate the cancer of corruption," Nuland said.