Ukraine's president was planning an emergency meeting of his military officials after a rebel rocket attack early Saturday morning killed 30 people in the port city of Mariupol.
President Petro Poroshenko intended to rush back home from Saudi Arabia, where he was to attend the funeral of King Abdullah, to coordinate a reaction to the attack, Ukrainian officials said.
Ukraine's top rebel leader announced Saturday that an offensive had begun on the strategically important port.
On Friday, the rebels rejected a peace deal and said they were going on a multi-prong offensive against the government in Kiev to vastly increase their territory. The move is upending European attempts to mediate an end to the fighting.
Mariupol, which lies on the Azov Sea, is the major city between mainland Russia and the Russia-annexed Crimean Peninsula. Heavy fighting in the region in the autumn raised fears that Russian-backed separatist forces would try to establish a land link between Russia and Crimea.
The RIA Novosti news agency cited eastern Ukrainian rebel leader Alexander Zakharchenko as saying an offensive has begun on Mariupol. He spoke Saturday as he laid a wreath at the site where at least eight civilians died when a bus stop was shelled in Donetsk, the largest rebel-held city in eastern Ukraine.
Rebel forces have positions within 10 kilometers (six miles) from Mariupol's eastern outskirts.
The Defense Ministry said in a statement there were three separate strikes from Grad multiple rocket launchers on Mariupol and surrounding areas.
"The area that came under attack was massive," Mariupol mayor Yuriy Khotlubei said in a video statement. "The shelling was carried out by militants. This is very clearly Russian aggression that has caused terrible losses for the residents of the eastern part of our city."
Defense and police officials said rockets struck homes, a kindergarten, a market and shops. There was no immediate report of how many people died in various locations.
A Ukrainian military checkpoint on a road leading out of the city toward rebel-held areas was also hit and one serviceman was killed, the Defense Ministry said.
Mariupol's city council urged residents not to panic and to ignore rumors that Ukrainian armed forces were planning to withdraw.
"On the contrary, all units are on fully battle-ready. Security measures in the city have been strengthened," the council said in a statement.
No armed separatist units have been noted moving toward the city, the statement added.
The U.N. human rights agency on Friday raised its estimate of the conflict's overall death toll to nearly 5,100 since April.
On Thursday, mortars rained down on the center of the separatist stronghold of Donetsk, hitting a bus and killing several bystanders. Rebel officials said 13 were killed in that attack. Monitors from the Organization for Security and Cooperation in Europe cited city morgue officials as saying they have received eight bodies.
On Jan. 13, a bus parked near an army checkpoint north of Mariupol was hit by a shell, resulting in the death of 13 people. Ukraine blamed the rocket attack on separatists.
The OSCE's special monitoring mission (SMM) in east Ukraine on Saturday expressed concern over the growing number of civilian casualties and called for restraint from all sides.
"The SMM strongly condemns the continuation of the fighting in residential areas. Using residential areas as firing positions attracts counter-firing to these areas, further endangering the lives of civilians," the mission said in a statement.
Clashes are taking place across the eastern regions of Donetsk and Luhansk, where separatist insurgencies emerged in April following Russia's annexation of Ukraine's Crimean Peninsula.
Fighting has also been intensifying recently for the government-held town of Debaltseve. Main roads to the town lie in separatist control and speculation is mounting that Ukrainian forces stationed there could soon be fully encircled.
Russia insists that it does not support the rebels, but Western military officials say the sheer number of heavy weapons under rebel control belies that claim.
A peace deal signed in September in the Belarusian capital of Minsk envisaged a cease-fire and a pullout of heavy weapons from a division line in eastern Ukraine. It has been repeatedly violated by both sides, and heavy artillery and rocket barrages have increased the civilian death toll in the last few weeks.
Foreign ministers from Russia, Ukraine, France and Germany agreed Wednesday to revive that division line, but fighting has continued unabated.
The Associated Press contributed to this report.