UPDATED: 12:22 p.m. ET

DONETSK, Ukraine — At least 30 people were killed on Saturday when rockets rained down on residential districts of the strategically significant eastern Ukrainian port city of Mariupol after Russian-backed rebels' rejection of peace talks and promise of a new offensive.

All of the dead were reported to be civilians. Two of them were children, according to Secretary of the Mariupol City Council Andriy Fedai, as cited by Interfax news service. Over 100 people were reported injured.

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It was not immediately clear which side carried out the attack. As with deadly attacks in recent days, Ukrainian and rebel officials blamed each other for the attack. But later in the day, in Donetsk, rebel leader Alexander Zakharchenko said the "attack on Mariupol has begun. We will have Debaltseve surrounded in days."

Debaltseve, a small city in far eastern Ukraine, remains under the control of the Kiev government.

More from #Donetsk rebel leader Zakharchenko: 'No better way to pay respect to those killed in Donetsk than by attacking #Mariupol.' — Christopher Miller (@ChristopherJM) January 24, 2015

Ukraine's Interior Ministry said in a statement that a 12-year-old girl was among those killed when a series long-range Soviet-made rockets called Grad, meaning hail, hit a market in an eastern district of the city shortly after 9 a.m. local time.

Smoke and flames rise above a burning building after shelling in Mariupol on January 24, 2015. Image: SERGEY VAGANOV/EPA

Ukraine President Petro Poroshenko said in a statement, "The bloody murders by pro-Russian terrorists of dozens of civilians that wounded nearly a hundred people in Mariupol is not just a terrorist act. This is a crime against humanity."

Mariupol police chief Vyacheslav Abroskin said in a Facebook post that the area had come under attack from separatist rebels in recent days.

Mariupol is the largest city in the war zone of eastern Ukraine that is still controlled by the Kiev government.

Ole Solvang, a researcher for Human Rights Watch, tweeted that he was in Mariupol and had counted 20 impact craters from the rocket attack, "many showing that the rockets came from east," where the rebels hold positions.

An Organization for Security and Cooperation in Europe (OSCE) report also documented evidence that indicated the rockets were fired from areas controlled by the rebels northeast and east of Mariupol.

A dead body surrounded by local residents lays on the ground in a residential area in Mariupol, Ukraine on January 24, 2015. Image: Sergey Vaganov/Associated Press

The American ambassador to Ukraine suggested rebels had launched the assault to show they were deadly serious about their promise on Friday of a fresh offensive to capture more territory in the east — a direct violation of an agreement between the warring sides last September.

Today's indiscriminate shelling of Mariupol part of an apparently Russian-backed general offensive in complete violation of Minsk agreements — Geoffrey Pyatt (@GeoffPyatt) January 24, 2015

Despite the evidence, the rebels' military office said its forces played no part in shelling, in comments carried by Russia's RBK. "This is misinformation and lies. The militias do not open fire in the direction of Mariupol, especially towards residential areas," it said.

Photos published by local news site 0629.com showed smoke rising from a crowded residential area and marketplace, and cars in flames.

This photo purports to show cars going up in flames after the rocket strike in #Mariupol. pic.twitter.com/IBBx5Nl32e via @SputnikATO — Christopher Miller (@ChristopherJM) January 24, 2015

Some reports indicated that the blasts had knocked out power and telecommunications.

This dashcam video purports to show the moment several of the missiles exploded and gives a rare perspective into what it is like to get caught in the middle of such an attack.

The following video is VERY GRAPHIC and MAY BE DISTURBING to some. Posted to the YouTube account of the far-right pro-Kiev Azov Battalion, whose base is inside the city, the graphic point-of-view video shows the gruesome aftermath of the attack at a densely populated marketplace.

In the video, raging flames and thick black smoke rise from destroyed shops lining bloodstained streets. At one point, a soldier gives medical attention to a badly injured person laying face down on the ground. A few frames later, the camera pans passed the body of a man whose head is covered and seeping blood as a woman weeps nearby. Another body is mangled so badly it is unrecognizable.

The southeastern city of about 500,000, sits on the Sea of Azov and connects southwestern Russia by highway to the Black Sea peninsula of Crimea that Russia annexed from Ukraine last March. Fighters here have expressed interest in creating a land bridge to the two areas, which would be extremely valuable for Moscow.

Burned cars destroyed in the shelling sit in a parking lot in Mariupol on January 24, 2015.

A massive rebel assault backed bay regular Russian forces in Mariupol last August prompted Ukraine to meet with Russian and rebel representatives in Minsk, Belarus on Sept. 5 and agree to a cease-fire. But that armistice was routinely violated, with at least 1,500 people killed since, and more than 5,000 people left dead in the war-torn east of the country since last April, according to the United Nations.

The attack in Mariupol comes days after shells struck a busy bus stop in the rebel stronghold of Donetsk, killing 13 people, and a similar attack the previous week at a Ukrainian-controlled checkpoint in nearby Volnovakha that also left 13 dead.

Kiev and the rebels, whose military rhetoric has intensified in recent days, also both blame each other for those attacks.