Thirty people were killed and more than one hundred injured in the attacks, which hit the city shortly after 0900 and at around 1300 on Saturday. Officials from the Organization for Security and Cooperation in Europe (OSCE) said Grad and Uragan multiple rocket launcher systems were used in the strikes, which crater analysis indicated were launched from two areas held by Russian-backed militants.

Journalists at the scene and experts who examined photographs of the craters left by the rockets also concluded that that the attacks were launched from the north-east and east, in areas controlled by the Russian-backed militants.

The rockets hit a residential area of eastern Mariupol. The OSCE said the nearest military target, a Ukrainian army checkpoint, was over 400 meters away from the area where the rockets hit.

Instead, the rockets rained a 30-second barrage of death on residential buildings, a market and a kindergarten, with five children were reported to be among those killed. Of the injured, more than 20 are said to have sustained life-threatening injuries, and the death toll from the attack is expected to rise.

The Ukrainian government ordered flags to be raised at half-mast on Sunday on the buildings of central government, local government and state institutions. It also ordered entertainment and sporting events on Sunday to be canceled, and changes to be made to radio and television broadcast schedules as a mark of respect for the dead.

The Russian-backed militants who have seized control of parts of the Ukrainian regions of Luhansk and Donetsk denied responsibility for the attacks. However, the attacks came just a day after the leader of the Donetsk People’s Republic militant organization, Oleksandr Zakharchenko, declared that his fighters were going on the offensive. On the same day as the attack, at a flower-laying ceremony for those killed in Donetsk last week in a mortar attack on a bus stop, Zakharchenko reportedly said that the deaths in Mariupol were a fitting response to the mortar attack.

He also said that the long-expected militant attack on Mariupol had begun, and that the government-held town of Debaltseve, in a salient thrust deep into militant-held territory, would be surrounded and attacked within days.

But after news of the scale of the attack on Mariupol began to spread, Zakharchenko denied that the militants had launched an attempt to take the city.

Meanwhile at the UN Security Council, Russia vetoed an attempt to pass a resolution condemning the rocket attack on Mariupol.

There is a growing body of photographic and video evidence that Russia has been supplying military equipment and troops to the militants that have seized control from the Ukrainian local authorities in parts of Luhansk and Donetsk regions.

Russia denies aiding the militants, but has admitted that some of its troops are fighting in Ukraine while “on vacation.”

The European Union and NATO on Saturday again called on Russia to end its support for the militant organizations in Ukraine’s east amid the sharp escalation of the fighting.

"Fighting in eastern Ukraine has sharply escalated, with indications of a large-scale offensive by Russian-backed separatists," NATO Secretary General Jens Stoltenberg said in a statement.

"This is in utter disregard of the ceasefire."

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