Ukraine is holding a day of national mourning on January 25, following a series of rocket attacks in the southeastern city of Mariupol that killed 30 people and injured more than 90 others.
Ukraine President Petro Poroshenko condemned the attack as "a crime against humanity," but he said Ukraine stands by a September peace deal signed in Minsk.
National flags were flown at half-mast and a minute of silence was observed at midday local time in memory of the victims.
Poroshenko also ordered the cancellation of sporting and cultural events on January 25.
Ukraine's Defense Ministry said the city had been targeted on January 24 with three separate strikes by Grad rockets fired from separatist-held areas.
The rockets hit a market and a residential district.
The rebels claimed Ukrainian forces were behind the attacks.
The Organization for Security and Cooperation in Europe (OSCE) said the rockets came from rebel-held areas.
The attacks have prompted widespread condemnation.
U.S. President Barack Obama said his administration was "deeply concerned" about the latest violation of the cease-fire.
Speaking at a press conference in New Delhi on January 25, Obama blamed "separatists with Russian backing, Russian equipment, Russian financing, Russian training and Russian troops" for the escalation of the situation in eastern Ukraine.
Obama said he'll keep trying to isolate Russia and he will examine options short of military conflict with Russia.
Poroshenko said on January 25 that Ukraine remains committed to a peaceful solution to the conflict that has killed more than 5,000 people since April.
Poroshenko told an emergency Ukrainian National Security and Defense Council meeting that "de-escalation" of the conflict was the priority for his government.
"We see no alternative to Minsk," he said in reference to the Belarusian capital where the often-broken cease-fire agreement with separatist rebels was signed on September 5.
Poroshenko said EU foreign ministers would meet on January 26 to discuss the crisis and the options for increasing pressure on Russia.
The rocket assault on Mariupol came a day after rebel commander Aleksandr Zakharchenko said the rebels would seek to seize control over the entire Donetsk region.
Footage on the Internet on January 24 showed Zakharchenko saying the separatists had begun an assault on Mariupol.
He also said the rebels would not initiate any more peace talks.
Meanwhile in Moscow, Russian Foreign Minister Sergei Lavrov -- speaking to U.S. State Secretary John Kerry on January 25 -- blamed the latest upsurge in violence in eastern Ukraine on Ukrainian government forces.
"Lavrov pointed out that an escalation of the situation is a result of Ukrainian troops crudely violating the Minsk agreements by constantly shelling residential settlements," the Russian Foreign Ministry said in a statement.
The statement also said Lavrov told Kerry Moscow was ready to do everything it can to encourage rival sides warring in east Ukraine to seek a peaceful solution to the conflict.
It said Kerry during the telephone conversation proposed to widen the group of international players trying to help peace talks between Kiev and rebels.
Separately, Lavrov told the European Union's foreign policy chief Federica Mogherini in a phone call on January 25 that Moscow wants the bloc to push Kyiv towards political dialogue with rebels.
In a Twitter message, Mogherini announced she was convening an extraordinary meeting of the EU’s foreign ministers to discuss the latest fighting in Ukraine on January 29.
Meanwhile, the French president's office said President Francois Hollande held talks with Poroshenko and European Council chief Donald Tusk to express his "very strong concern" over the surge in violence in eastern Ukraine,
It added that Hollande was also due to speak with Russian President Vladimir Putin and German Chancellor Angela Merkel on January 26 to discuss the escalation of fighting.
German government spokesman Steffen Seibert said Merkel called the attack on Mariupol "a clear and totally unjustifiable violation of the cease-fire" in telephone calls with the presidents of Ukraine and Russia on January 25, and asked Putin to prevent further escalation.
And the Serbian chairmanship of the OSCE announced that the organization's ambassadors will hold a special meeting in Vienna on January 26 to discuss the need for further peace consultations between representatives of Ukraine, Russia, the OSCE, and the other signatories of the Minsk agreement.