By Anthony Deutsch

AMSTERDAM Ukrainians living in Russian-occupied territory are being forced to assume Russian citizenship or face harsh retaliation, including possible deportation or detention, U.S.-backed research published on Wednesday said.

Yale University researchers said that as part of a plan by Moscow to assert authority over Ukrainians, residents of the Luhansk, Donetsk, Kherson and Zaporizhzhya regions are being targeted by a systematic effort to strip them of Ukrainian identity.

A series of decrees signed by Russian President Vladimir Putin compel Ukrainians to get Russian passports, in violation of international humanitarian law, the report said.

The Kremlin has consistently denied allegations of war crimes in Ukraine by forces taking part in a "special military operation" it says was launched to "de-Nazify" its neighbour and protect Russia.

Ukrainians in occupied territory who do not seek Russian citizenship "are subjected to threats, intimidation, restrictions on humanitarian aid and basic necessities, and possible detention or deportation – all designed to force them to become Russian citizens," the report said.

"What is concerning here is that it represents, basically, a violation of the Hague and Geneva Conventions," Executive Director Nathaniel Raymond of the Humanitarian Research Lab at the Yale School of Public Health, told Reuters. "It is very widespread and very ongoing."

Ukrainians in areas under Russian control have no choice but to accept a Russian passport if they want to survive, or they face potential detention and, as the team has documented, deportation into Russia if they fail to comply," Raymond said.

Responsibility lies at the Kremlin with President Putin, who has been indicted by the International Criminal Court (ICC) for the war crime of deporting Ukrainian children and occupation authorities, he said.

The Kremlin has said that the ICC's decision to issue an arrest warrant for Putin is a sign of the "clear hostility" that exists against Russia and against Putin personally.

The report was released as part of the Conflict Observatory program, with the support of the U.S. State Department and conducted by research partner the Yale School of Public Health's Humanitarian Research Lab.

(Reporting by Anthony Deutsch; Editing by Giles Elgood)