Thirty civilians were killed and102 wounded in missile attacks Saturday on Mariupol, an industrial center and seaport in southeast Ukraine, officials said.
The Grad missiles struck a day after Donetsk region pro-Russia separatist leader Alexander Zakharchenko said his forces were launching a wide-ranging offensive.
"This is a crime against humanity to be tried by the Hague tribunal," Ukrainian President Petro Poroshenko said in a statement condemning the attack.
U.S. Secretary of State John F. Kerry also denounced the assault. "It is reprehensible that the separatists are publicly glorifying this and other offensives," he said in a statement. He said the attacks were "aided and abetted by Russia's irresponsible and dangerous decision to resupply" the separatists in recent weeks.
About 60 missiles landed in eastern Mariupol in a pair of attacks, one in the morning and the other about four hours later, Donetsk regional deputy police chief Ilya Kiva said.
"Several streets, many residential houses and two food markets were hit. This being a Saturday morning, many people were at home or outside," Kiva said in a phone interview. "Whole families died either in their apartment or in the street strolling or going to the market."
Kiva said he had no doubt that it was intentional, given the accuracy of the second salvo. "The nearest Ukrainian army checkpoint outside the city is about [3 miles] from the place, and the terrorists know well where it is, because they have already shot at it many times in the last three days."
The northern Donetsk regional towns of Debaltseve, Sverdlovsky, Maryinka and Kurakhovo were also bombarded, Kiva said. "Altogether there were over a dozen wounded in those places; the comparatively low number of casualties there can be explained by the fact that people are simply afraid to go out into the streets anymore," he said.
The armed conflict in eastern Ukraine began in April after Russia's annexation of Crimea. Groups of gunmen led by Russian mercenaries seized government offices, police and security stations and distributed arms to residents unhappy about the downfall of President Viktor Yanukovich, who was ousted in February after massive street protests in the capital, Kiev, turned violent.
Since then, Russia has reportedly equipped separatists in the Donetsk and Luhansk regions with Grad missiles, antiaircraft systems, tanks and armored vehicles and has allowed hundreds of volunteers to cross into Ukraine to fight for the fast-growing separatist army.
In August, NATO accused Russia of sending regular troops into Ukraine to help a separatist offensive. Last week, Poroshenko charged that about 2,000 Russian troops aided by 200 tanks had entered Ukraine to join 8,000 Russian troops and 300 tanks already based in the volatile eastern regions.
The shaky truce that had lasted since September was broken by intense fighting earlier in the week. Separatist forces managed to win control of a few key buildings at Donetsk airport, scene of the fiercest fighting since May.
Both sides declared that they had inflicted heavy casualties on the other.
On Friday, Zakharchenko, prime minister of self-proclaimed Donetsk People's Republic, said that separatist forces in Donetsk and Luhansk were beginning a five-direction offensive.
"Today I don't see any sense in conducting peace talks," Zakharchenko said at a meeting with local university students, after eight people were killed and 13 wounded in a mortar attack on a bus stop in Donetsk on Thursday. "Ukraine perceived our kindness like a weakness. When declaring a truce we saw that Ukraine was deceiving us."
Zakharchenko said that Ukraine had more human and technical resources but that his "men are happy to go into battle."
Ukraine's Foreign Ministry blamed Russia for civilian deaths in Mariupol, Donetsk and other towns.
"The shelling occurred … against the backdrop of massive deployment of regular Russian army units, weapons and hardware in the territory of the Donetsk and Luhansk regions," read a statement posted on the ministry's official website Saturday. "Russia, which continues to support the terrorist activities of the illegal armed groups in Donbass and whose servicemen take direct part in combat actions against Ukraine, bears full responsibility for the innocent victims in Volnovakha, Debaltseve, Donetsk, Mariupol and many other towns and villages of Ukraine."
Ukrainian Defense Minister Stepan Poltorak said that his nation's armed forces destroyed four Grad systems out of six that delivered the deadly strike on Mariupol and that the army was transporting more troops to the area.
Hromadske TV, an independent station, aired video from Mariupol showing cars and kiosks burning, black smoke billowing and streets covered with rubble. Bodies were seen lying in the streets while the wounded were evacuated by ambulance. Women screamed for help.
"The terrorists are preparing for their offensive and are aiming to sow panic in peaceful Ukrainian regions, so we must brace for more attacks," Kiva said.
The Ukrainian conflict has claimed more than 5,000 lives, the United Nations said last week.
Poroshenko said of Saturday's attack, "The bloody murder by pro-Russia terrorists of dozens of peaceful civilians and wounding of [about] 100 Mariupol residents is not just a terrorist act" but a crime against humanity.
"Any help to the gunmen, supply of weapons, hardware and well-trained personnel is support of terrorism; that is obvious to the entire world, isn't it?" he said.