Heavy fighting has erupted between Ukrainian forces and Russian-backed separatists near the rebel stronghold of Donetsk, in an escalation that threatens to rip apart a tenuous internationally brokered peace deal.

The Ukrainian Armed Forces General Staff said that a June 3 rebel offensive near the eastern Ukrainian towns of Maryinka and Krasnohorivka, west of Donetsk, forced it to use artillery that had been removed under the Minsk II agreement as part of an effort to create a demilitarized zone.

The agreement has served as the basis for a shaky cease-fire since February.

Ukraine's military said it warned its "international partners" about its redeployment of the heavy weaponry, which it said was necessary to repel a separatist offensive that involved 1,000 fighters as well as tanks, mortars, and artillery.

"For the purpose of appropriate response, we were forced to use heavy artillery," military spokesman Oleksiy Mazepa said on television channel 112.

The Ukrainian president's office said that two Ukrainian soldiers were killed and 30 wounded in the fighting.

The self-styled Defense Ministry of the separatist Donetsk People's Republic (DNR) accused Ukrainian forces of initiating the violence, which broke out at around 3 a.m. local time on June 3, saying Kyiv was attempting to "disrupt" the Minsk accords.

A spokesperson for the separatists said 15 fighters and civilians were killed in rebel-controlled areas as a result of the fighting.

Separatists also said that electricity supplies had been cut off, leaving hundreds of miners trapped underground in two mines.

Video footage from RFE/RL's Current Time television program indicates that the Tekstilshik district near Maryinka on the Donetsk outskirts came under artillery fire.

The June 3 footage shows firefighters and emergency responders working at the site of a charred, bombed-out market, as patches of flames continued to burn.

The fighting lasted for around 12 hours. Ukrainian Defense Minister Stepan Polotorak told reporters later that "for the moment, storming operations have been halted.

The Kremlin quickly took the side of the separatists, accusing the Ukrainian military of "provoking" the new fighting and called for the unconditional implementation of the Minsk peace agreement.

"Naturally Moscow is very closely following and feeling extreme concern in relation to the provocative actions of the armed forces of Ukraine that, as far as we can judge, largely provoke the situation," Kremlin spokesman Dmitry Peskov told reporters on June 3.

He added that Russia expected "the unconditional fulfillment of the Minsk agreements."

The United States, meanwhile, said the onus was on Moscow to ensure the peace accords.

"Russia bears direct responsibility for preventing these attacks and implementing a cease-fire," U.S. State Department spokeswoman Marie Harf told a news briefing in Washington on June 3.

She said Washington was disturbed by reports of an offensive by Russian-backed separatists and that such attacks could lead to additional costs for Moscow.

Ukraine and the separatists agreed to pull back heavy weaponry from the front lines in eastern Ukraine's Donbas region under the Minsk cease-fire, though sporadic fighting has continued since the deal was reached in the Belarusian capital.

Kyiv, NATO, and Western governments accuse Russia of providing arms, personnel, and training to the separatists in the conflict, which the United Nations estimates has killed more than 6,400 people since April 2014.

Moscow has repeatedly rejected the accusation despite mounting evidence of such involvement.

It says any Russian citizens fighting alongside the rebels are volunteers acting independently.

With reporting by Reuters, Interfax, AFP, and AP