Kiev: Indiscriminate rocket fire slammed into a market, schools and homes on Saturday in the eastern Ukrainian city of Mariupol, killing at least 21 people, authorities said.
Ukraine's top rebel leader announced that an offensive had begun on the strategically important port.
The report came a day after 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 attempt\s 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 today 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."
The Donetsk region prosecutor's office said in a mid-afternoon statement that 21 people had been killed. 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.