Tusk's comments come a day after German Chancellor Angela Merkel said she was "horrified" by the suffering caused by Russian airstrikes.
"Let me add a comment on Russia, whose actions in Syria are making an already very bad situation even worse," Tusk said after meeting Georgian Prime Minister Giorgi Kvirikashvili in Brussels.
"As a direct consequence of the Russian military campaign, the murderous Assad regime is gaining ground, the moderate Syrian opposition is losing ground and thousands more refugees are fleeing towards Turkey and Europe."
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Former Polish premier Tusk -- known for his outspoken comments in the past on Russia's role in the Ukraine crisis -- was speaking as the the 28-nation European Union struggles to deal with a huge influx of refugees and migrants, mostly from Syria.
More than one million migrants and refugees arrived in Europe last year.
The Kremlin on Tuesday rebuked Merkel for saying -- referring to air strikes including those carried out by Russia -- that "we are horrified in the face of this human suffering."
Russia insists it is only targeting the Islamic State group in Syria, as Western and Arab countries are already doing. But the West says Moscow is also hitting moderate rebel groups in Syria and has caused civilian casualties.
NATO chief Jens Stoltenberg meanwhile on Tuesday said the alliance would seriously consider calls by Germany and Turkey to help tackle the migrant crisis in the Mediterranean, as well as criticising Russia's involvement.