Secretary of State John Kerry condemned as "grotesque" on Thursday the distribution of leaflets in eastern Ukraine that appeared to call on Jews to register with separatist, pro-Russian authorities.
Though purported authors of the flier described it as a crude attempt to discredit them, Kerry said: "Notices were sent to Jews in one city indicating that they had to identify themselves as Jews ... or suffer the consequences.
"In the year 2014, after all of the miles traveled and all of the journey of history, this is not just intolerable; it's grotesque. It is beyond unacceptable," he said in Geneva, where he met Russian, Ukrainian and EU counterparts to draw up a four-way agreement to work to defuse the crisis in eastern Ukraine.
Kerry said Russian Orthodox Church members in Ukraine had also received threats "that the Ukrainian Orthodox Church was somehow going to attack them in the course of the next days."
"That kind of behavior, that kind of threat, has no place," he said.
Kerry said all parties at the Geneva meeting had condemned such threats and intimidation.
The origin of the leaflets in Donetsk was unclear.
On Wednesday, local news site Novosti Donbassa quoted unidentified members of Donetsk's Jewish community as saying three masked men handed them out near the city's synagogue on Monday, when Jews were celebrating the start of Passover.
Purporting to be issued by the Donetsk People's Republic, a pro-Russian group which last week took over public buildings and wants to end rule by the new Ukrainian government in Kiev, the leaflet said all Jews aged over 16 must register with a "commissar" at the regional government headquarters by May 3.
Failure to comply would lead to deportation and the "confiscation of property".
Its preamble explained that action was being taken because Jewish leaders had supported the "junta" which took power in Kiev after the overthrow of the Moscow-backed president.
Kirill Rudenko, a spokesman for the People's Republic of Donbass, said the statement was "complete rubbish": "We made no such demands on Jews," he said. "We have nothing against Jews.
"This is just another attempt to tarnish our image ... It is a crude forgery."
Once home to a large Jewish population that was devastated by the Holocaust, Ukraine has seen a rise in attacks on Jews and on synagogues since unrest began five months ago.
Some Ukrainian nationalist groups which took part in the uprising in Kiev have been blamed for fanning anti-Semitic sentiment. Anti-Semitism is also apparent among some Russian nationalists.
U.S. State Department spokeswoman Marie Harf said in Washington that the United States was still trying to determine who was behind the leaflets and added: "We take any anti-Semitism very seriously."
An influential U.S. congresswoman called the leaflet episode
"an unacceptable escalation of the crisis in Ukraine and cause for both grave concern and immediate action".
Nita Lowey, the senior Democrat on the U.S. House Appropriations Committee, said in a letter to Kerry that the singling out of Jewish communities for scrutiny and possible punishment "reeks of age-old anti-Semitic policies".
"All of the parties involved in the ongoing crisis in Ukraine must understand in no uncertain terms that the world community will not tolerate such contemptible and atrocious behavior," she said.
U.S. Senator Charles Schumer, the No. 3 Democrat in the Senate, said Russian President Vladimir Putin "has accused the Ukrainians recently of being anti-Semitic, but now it is pro-Russian forces that are engaged in these grotesque acts".
He urged Putin to denounce the anti-Semitic acts and use his influence to stop them.
Reporting by Arshad Mohammed in Geneva, Gabriel Baczynska in Donetsk, Alastair Macdonald in Kiev and David Brunnstrom in Washington; Editing by Tom Heneghan and Eric Walsh