President Petro Poroshenko said there are a "significant" number of casualties after missiles hit Ukraine's military headquarters and a residential area in the eastern city of Kramatorsk.
The government later announced at least 15 people had been killed and more than 60 wounded in the February 10 attack.
Poroshenko told parliament on February 10 that the attack hit the headquarters of the "Antiterrorist Operation," as Kyiv calls the fight against pro-Russian separatists in eastern Ukraine, and several civilian houses. He said both civilians and military officers were injured.
The Kyiv-controlled Donetsk regional administration said the rockets were fired from the separatist-held area of Horlivka, which is about 50 kilometers from Kramatorsk.
Eduard Basurin, a self-proclaimed separatist defense official, said the rebels "did not strike Kramatorsk" and contended that it was out of their range.
On Twitter, U.S. Ambassador to Ukraine Geoffrey Pyatt wrote: "We condemn Russian-backed separatist shelling of civilian areas #Kramatorsk."
The missile attack came after a month of intense fighting that has prompted a diplomatic push for peace and added to Western pressure on Russian President Vladimir Putin, who the United States and NATO have called upon to abandon support for the rebels and withdraw troops and arms from Ukraine.
German Chancellor Angela Merkel and French President Francois Hollande, who brought a peace proposal to Kyiv and Moscow last week, may meet with Putin and Poroshenko in the Belarusian capital, Minsk, on February 11, but prospects for the talks were uncertain.
Forces on both sides in the conflict claimed territorial gains ahead of the proposed talks.
Ukraine’s volunteer Azov battalion said on social media on February 10 that it has captured several villages northeast of Mariupol, pushing Russian-backed separatists away from the strategic Azov Sea port city.
Rebel military spokesman Eduard Basurin said late on February 9 that separatist forces have surrounded the town of Debaltseve, a transport hub, cutting it off from a major highway.
WATCH: Rockets struck near a soup kitchen in the separatist-held town of Pervomaisk in eastern Ukraine on February 10. People dived for cover before rushing to take shelter in the cellar. (Reuters)
Most of Debaltseve's 25,000 residents have been evacuated.
Debaltseve has been the site of some of the fiercest fighting in the recent escalation of the conflict, which has killed more than 5,350 people since April.
The United States has said the escalation is the result of a "Russian-backed offensive" by rebels in violation of an agreement signed in Minsk in September on a cease-fire and steps toward peace.
Ukrainian military spokesman Andriy Stelmakh said on February 10 that there had been "an increase in attacks by the enemy on Ukrainian positions" in eastern Ukraine during the previous 24 hours.
He said seven Ukrainian soldiers were killed and 23 wounded.
Both Putin and German Foreign Minister Frank-Walter Steinmeier have raised doubts about the proposed four-way summit in Minsk, suggesting it was not yet certain it would take place.
Steinmeier said much work must be done on “open points” before the meeting could take place.
Russian President Vladimir Putin also has said the Minsk summit would not take place if the four leaders fail to agree on a "number of points" beforehand.
Denis Pushilin, a leader of pro-Russian separatists in Ukraine's Donetsk province, was in Minsk on February 10 for an expected meeting of a Contact Group that includes representatives of Ukraine, Russia, and the OSCE.
In Washington, U.S. President Barack Obama says he is considering the option of sending lethal defensive weapons to Kyiv "if diplomacy fails" to end the war.
Speaking after talks with German Chancellor Angela Merkel on February 9, Obama said he hoped that a diplomatic solution was still possible.
But he also said it is clear that Russia has violated the commitments it made on Ukraine under the deal reached in Minsk in September.
Obama said: "It's clear that they violated just about every commitment they made in the Minsk agreement. Instead of withdrawing from eastern Ukraine, Russian forces continued to operate there, training separatists and helping to coordinate attacks. Instead of withdrawing its arms, Russia has sent in more tanks and armored personnel carriers and heavy artillery."
Putin has said he will not give in to Western ultimatums.
In a telegram sent to Russian diplomats on February 10, Putin said that “no matter how much pressure is put on us,” Russia “will continue to pursue an independent foreign policy, to support the fundamental interests of our people and in line with global security and stability.”
Earlier on February 9, Ukrainian military spokesman Andriy Lysenko accused Russia of sending an additional 1,500 troops and more than 300 military vehicles into eastern Ukraine during the previous two days.
Last month, Ukrainian President Petro Poroshenko said that some 9,000 Russian troops were in Ukraine with 500 military vehicles, including tanks, Grad missile launchers, and heavy artillery.
Russia denies it has sent troops or weapons to Ukraine, despite mounting evidence.