Worst east Ukraine shelling in a month
No doubt
In one 40-vehicle convoy, "19 were large trucks - Kamaz type, covered, and without markings or number plates - each towing a 122 mm howitzer and containing personnel in dark green uniforms without insignia," the watchdog said in statement.
Ukraine said it had no doubt the new troops were Russians.
"Although the OSCE did not specify to whom the equipment and soldiers belonged, the Ukrainian military has no doubt of their identity," said military spokesman Andriy Lysenko.
"The past week was characterized by an increase in the intensity of shelling and the transfer of additional force: ammunition, equipment and personnel, to terrorist groups," Lysenko said.
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Reuters reporters in rebel-held Donetsk said intense shelling by heavy artillery continued throughout the night and into the early hours, and then picked up again later on Sunday morning. The shelling could be heard in the center of the city, which had a pre-conflict population of more than 1 million.
"There have been rumors for a while that one of the sides is getting ready to break the ceasefire and go on the offensive," local businessman Enrique Menendez said, describing Saturday's shelling as a "night of wrath".
Large clouds of black smoke could be seen over the ruins of the airport, which is still under government control but which the separatists are seeking to seize.
Lysenko said three Ukrainian soldiers had been killed in the past 24 hours and a further 13 injured. The media service for the military operation said two policemen and one civilian had died in shelling on Sunday.
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Worst east Ukraine shelling in a month