Pro-Russian rebels on Saturday launched an offensive on the key Ukrainian port city of Mariupol, in an escalation the European Union foreign policy chief warned could lead to a further "grave deterioration" of EU-Russian relations.
Ukrainian authorities said at least 30 people were killed and more than 80 were wounded when rockets hit a market and residential buildings in Mariupol.
The attack came just hours after the rebel leader in the Donetsk region said there would be no further peace talks with Kyiv.
Ukrainian President Petro Poroshenko responded to the rocket fire with a vow to protect Ukrainian territory and a call for an emergency meeting of his security council. He spoke about the situation in a telephone call with U.S. Vice President Joe Biden on Saturday.
A White House statement said Biden condemned the attacks and violence he said was initiated by Russia-backed separatists. The statement also said the two expressed grave concern over Russia's disregard for its commitments under the September Minsk agreement.
Latvia, which currently hold the European Union's rotating presidency, called for the bloc to hold an emergency meeting with its foreign ministers to discuss the situation.
NATO Secretary-General Jens Stoltenberg issued a statement saying the offensive "is in utter disregard of the cease-fire." He said Russian troops have been supporting rebel operations with air defense systems, surface-to-air missiles and "advanced multiple rocket launcher systems."
Both Stoltenberg and EU foreign policy chief Federica Mogherini urged Russia to stop providing the rebels with military and financial support. Mogherini called on Russia to use its influence to halt the offensive.
U.S. Secretary of State John Kerry issued a statement Saturday, saying if Russia does not end its support for separatists and "withdraw all weapons, fighters and financial backing," U.S. and international pressure "will only increase." Kerry called the Mariupol assault "horrific" and said he joined his European counterparts in condemning it in the strongest terms.
U.N.Secretary-General Ban Ki-moon also condemned the attack. He said the rockets appeared to have been launched indiscriminately into civilian areas, which would constitute a violation of international humanitarian law.
Dozens of mourners held a minute of silence and lit candles in Kyiv's Independence Square on Saturday evening to commemorate those killed in Mariupol.
One Kyiv resident, Anna Murina, who attended the solemn gathering, said it was sad to see civilians killed by those supported by a neighboring country that many in Ukraine view as a close relative.
Mariupol sits on the Azov Sea between mainland Russia and the Russia-annexed Crimean Peninsula.
On Friday, Russian-backed Alexander Zakharchenko, head of the self-declared Donetsk People's Republic, said his forces would attack government troops until reaching the borders of the Donetsk region. He said his fighters were advancing against Ukraine combat troops after seizing control of a long-contested airport Thursday.
Separatists currently control key territory in the Donetsk and Luhansk regions near the Russian border, after launching a rebellion nine months ago against Ukraine rule.
Zakharchenko was quoted in Russian media as saying he saw "no point" in further four-party peace talks. He told Russia's Interfax news agency that the Minsk peace talks format agreed on last year was "a mistake" that "we will not make again."
Separately, Interfax quoted rebel military official Eduard Basurin as saying more than 750 Ukrainian troops had been killed in recent days in the battle for the now-destroyed Donetsk airport.
He said more than 100 Ukraine tanks and other fighting vehicles had also been destroyed. The rebel claims have not been independently confirmed. U.N. officials, relying on reports from the conflict zone, said they had tallied 262 deaths over nine days.
In Moscow on Friday, Russian President Vladimir Putin blamed Ukraine for the latest escalation of violence. He accused Kyiv of launching an offensive using artillery and aircraft that he said had killed or wounded dozens of people, including women, children and the elderly.
Doctors Without Borders said Friday that the situation for civilians caught in the conflict zone was "dire." The international medical charity said doctors close to the front line were "struggling to treat the wounded with dwindling supplies," while heavy fighting was preventing medical teams from reaching the hardest-hit areas.
The United Nations human rights agency said Friday that the death toll from the conflict had passed 5,000. But spokesman Rupert Colville said the death toll could be far higher.
In Washington on Friday, State Department spokeswoman Jen Psaki said the U.S. remained deeply concerned by the increasing violence and bloodshed in eastern Ukraine, which she said had resulted from a surge in Russia-backed separatist attacks against the cease-fire line. She said separatists had carried 1,000 attacks since early December.
Psaki said Russia was actively supporting the separatists by supplying them with heavy weaponry and vehicles, including tanks, armored personnel carriers and heavy artillery, as well as providing military personnel for exercising ongoing tactical support.
She again called on Russia to stop the flow of heavy weapons, fighters and advisers, restore Ukraine’s control along its side of the international border and allow monitoring by the Organization for Security and Cooperation in Europe along both sides of the border.
Some material for this report came from AP, AFP and Reuters.