Ukraine’s Interior Ministry says at least 30 civilians were killed and scores more were wounded on January 24 when the government-held port city of Mariupol in the southeast of the country was targeted by heavy barrages of Grad rockets as part of a Russian-backed separatist offensive.
The local mayor’s office said a crowded market was hit and other buildings were in flames in a southeastern residential district of Mariupol near roads that have come under attack from Russian-backed separatists in recent days.
Ukrainian authorities said another 97 people were injured in the attack.
In a statement, Ukrainian President Petro Poroshenko called the attack "a crime against humanity and said he was cutting short a trip to Saudi Arabia to return to Ukraine to chair a meeting of the country's top security body to discuss the attack.
Poroshenko's statement also vowed to deliver a "full victory" over the rebels.
The Ukraine mission of the Organization for Security and Cooperation in Europe (OSCE) confirmed in a statement that the rockets were fired from the territory controlled by pro-Russian rebels.
The pan-European security body said its inspection team concluded that "the Grad rockets originated from a northeasterly direction... and the Uragan rockets from an easterly direction, both controlled by the 'Donetsk People's Republic'."
In an earlier statement, the OSCE said that it viewed "the shelling in Mariupol... as a reckless, indiscriminate and disgraceful attack."
In Brussels, the European Union denounced the attack and warned that the escalation in fighting will harm EU-Russia relations.
The escalation "would inevitably lead to a further grave deterioration of relations between the EU and Russia," EU foreign affairs chief Federica Mogherini said in a statement.
"Residential areas of the city of Mariupol have been fired upon today from separatist held territory, which has cost the lives of at least fifteen civilians, wounding many others and terrorising the innocent population," the statement said.
Ukrainian military spokesman Andriy Lysenko told a news conference in Kyiv that "at least three Grad launchers were used in the attack, which were aimed right at civilians, at residential areas."
The Interfax news agency reported that Prime Minister Arseniy Yatsenyuk had called on the UN Security Council to hold an urgent meeting after the attack.
Meanwhile, Aleksandr Zakharchenko, a pro-Russian rebel leader in eastern Ukraine was quoted by Russia's RIA news agency as saying separatists had launched an offensive on Mariupol on January 24.
Zakharchenko was quoted as saying, "Today an offensive was launched on Mariupol. This will be the best possible monument to all our dead."
RIA said Zakharchenko was speaking as he laid a wreath at the site where at least eight civilians died on January 22 when a bus stop was shelled in Donetsk, the largest rebel-held city in eastern Ukraine.
Zakharchenko on January 23 announced a multi-pronged rebel offensive.
He said separatists “won’t make an effort to talk about a cease-fire” in the nine-month-old conflict, which has killed more than 5,000 people.
Zakharchenko also reportedly said he would order his forces "to take no prisoners" in the future.
Pro-government volunteers in the Azov regiment who are defending Mariupol vowed that they would not retreat from the city.
PHOTO GALLERY: Rockets Target Mariupol (Contains Disturbing Images)
German Foreign Minister Frank-Walter Steinmeier on January 23 accused Zakharchenko and pro-Russian separatist fighters of "nothing but warmongering" and insisted that rebels and Ukrainian forces begin withdrawing their heavy weapons away from the frontline in the conflict.
U.S. Secretary of State John Kerry has accused the separatists of attempting "a blatant land grab" in eastern Ukraine.
The separatist offensive comes after government forces on January 21 withdrew from fortified positions they had defended for months in and around the terminal buildings of the Donetsk airport.
It also comes after Ukrainian President Petro Poroshenko said on January 21 that Russia's regular army had deployed 2,000 troops into eastern Ukraine in recent days.
Poroshenko told the World Economic Forum in Davos, Switzerland that the Russian military force in eastern Ukraine now numbers more than 9,000 soldiers with more than 500 tanks, armored personnel carriers, and heavy artillery pieces.
NATO Secretary-General Jens Stoltenberg told the Reuters news agency on January 23 that the alliance has noted a "substantial increase in Russian heavy equipment" in Ukraine, including "tanks, armored vehicles, artillery, [and] advanced air defense systems."
Stoltenberg said he was prepared to meet with Russian Foreign Minister Sergei Lavrov on the sidelines of a Munich security conference in early February.
Russian President Vladimir Putin blamed Kyiv for the escalation of fighting, claiming that government forces were conducting "large-scale military action" against the separatists.
Russia denies involvement in the conflict despite what Kyiv and NATO say is incontrovertible evidence it has sent troops into eastern Ukraine and provided the rebels with sophisticated weapons.