The accusation, which was not backed by evidence of ethnic or sectarian killings, came as Kurdish fighters reached the outskirts of a strategic Islamic State-held town on the Turkish border.
Thousands of refugees, most of them Sunni Muslim Arabs, have fled to Turkey in the past two weeks as the main Kurdish fighting force known as the YPG has pushed to dislodge IS militants from Tal Abyad, aiming to sever a key supply line for the extremists' nearby de facto capital.
The Kurdish advance, coming under the cover of intense US-led coalition airstrikes in the area, has aggravated sectarian and ethnic tensions in the region.
"YPG forces ... Have implemented a new sectarian and ethnic cleansing campaign against Sunni Arabs and Turkmen under the cover of coalition airstrikes which have contributed bombardment, terrorizing civilians and forcing them to flee their villages," the statement issued by rebel and militant groups said.
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The 15 groups, including the powerful ultraconservative Ahrar al-Sham and Jaish al-Islam, said the alleged "cleansing" is concentrated in the northeastern predominantly Kurdish province of Hassakeh and in Tal Abyad. They said it was caused by the Kurdish advance and U.S.-led airstrikes.
YPG spokesman Redur Khalil strongly refuted the claims, calling the groups making them "bankrupt."
"Preserving people's dignity is among our priorities, and it is for that that we are offering our blood," he wrote on his official Facebook page Monday. He said the "unjust accusations were meant to market for the Islamic State group and cover up its crimes."