A Russian armored vehicle is seen in front of a Ukrainian army base at the town of Perevalne, Ukraine, on March 14, 2014.

BERLIN — In eastern Ukraine, among pro-Russian separatists, the notion that elite American fighters are prowling the backroads and slag heaps of their region is oft-repeated. After first surfacing in March, the rumors sounded like the sort of paranoid fantasies created in a war zone where anti-Americanism is rampant.

But now the rumors are being repeated in Germany’s capital — and resonating. That alone may count as a victory for Russian propagandists, even if there are no American mercenaries. The White House says there are not.

Bild am Sonntag, a tabloidlike newspaper that occasionally breaks major stories on the German government, is reporting that German intelligence has told Chancellor Angela Merkel’s office that it had unconfirmed reports that 400 Americans appear to be aiding the interim Ukrainian government in its fight against pro-Russian separatists. According to Bild, the German intelligence agency cited U.S. intelligence officials as its source.

The report, which appeared Sunday, has since been repeated by many German news outlets. The allegation that the information was presented to the chancellor’s office in a weekly briefing in April lends it gravitas. That such reports in Bild on more than one occasion have proved true enhances its credibility. The chancellor’s office and the German intelligence service have declined to either confirm or deny, a development that leaves an atmosphere of doubt in a country where tensions are rife about just how angry Germany should be at Russia’s actions in Ukraine — fueled in no small part by German reliance on Russian natural gas and oil and the extensive business ties between the two nations.

“We’ve told Bild and other outlets that these stories are nonsense,” Caitlin Hayden, the national security spokeswoman for the White House, said Wednesday. “The company in question has been on record denying it for almost two months.”

The Russians couldn’t hope for more.

Ukrainian experts and officials note that Russia has been waging a decadelong propaganda war to sour Ukrainians on the government in Kiev, and this report fits right in to Russia’s hopes to reduce international pressure on it. Russia has been widely accused of funding, supporting and supplying officers to the pro-Russia separatists in the Donetsk Oblast, or region, of eastern Ukraine. To be able to show that the United States, even in the form of mercenaries and not official military personnel, are active on behalf of Kiev would to many further justify Russian actions.

According to Bild, the German intelligence agency Bundesnachrichtendienst, or BND, said the mercenaries were hired from Academi, an international security firm that rose from the scandal-ridden ashes of Blackwater, the now defunct company that made headlines worldwide for its alleged questionable ways in Iraq, where its contractors were accused, among other things, of killing 17 Iraqi civilians in a shooting spree in Baghdad’s Nisouri Square.

Bild reported that the BND said that the 400 Academi “elite” soldiers are thought to have been part of operations against pro-Russia separatists around Slovyansk, the eastern Ukrainian city that has been the scene of fierce fighting between separatists and Ukrainian security forces.

Academi officials could not be reached for comment, but in March, when the rumors first began circulating, they posted an official denial on their website. At that time, the international and Ukrainian media focus was on Crimea, which had been occupied by Russian military forces in late February and had just voted in a questionable referendum to secede from Ukraine and join Russia.

“Some irresponsible bloggers and an online reporter have recently posted rumors that ACADEMI employees (operating under the name of Blackwater) are present in Ukraine,” the statement said. “They are not and ACADEMI has no relationship with any entity named Blackwater or with the former owner of Blackwater, Erik Prince. Such unfounded statements combined with the lack of factual reporting to support them and the lack of context about the company, are nothing more than sensationalistic efforts to create hysteria and headlines in times of genuine crisis.”

An American mercenary force of 400 would be a significant factor in fighting in the region. Ukrainian estimates place as many as 800 pro-Russia separatist fighters in Slovyansk, and no more than several thousands in the region. The pro-Russia fighters are described as a collection of regional and Russian mercenaries, allegedly led by Russian spetsnaz, or special forces.

There are an estimated 15,000 Ukrainian military in the east of the country, though military experts note that they are primarily focused on guarding the border with Russia. Serhiy Zhurets, the director of the Center for Army, Conversion and Disarmament Studies, a research center in Kiev, noted that the Ukrainian constitution prohibits the military from engaging in the internal conflict. Instead, the fighting has fallen to police and special forces of Ukraine’s intelligence service.

The German report comes against steady claims by separatist fighters that they’ve heard American voices on Ukrainian military radios. The rumors are repeated as evidence that the government in Kiev is nothing more than a Western puppet regime. Their reasoning is that American warriors are trying to suppress the pro-Russia movement, just as they toppled the pro-Russian government of President Viktor Yanukovych by running the protests at Maidan, or Independence Square, in Kiev.

The Russian government news agency Ria Novosti reported on April 7 that elite Academi warriors were wearing Ukrainian Sokol (special forces) and police uniforms and engaged in fighting.

Suddeutsche Zeitung columnist Hubert Wetzel on Wednesday described the rumors as potentially devastating for the Western cause in Ukraine. His column made a point of noting that even the report of the report to the chancellor’s office was still a rumor. But he noted that if confirmed, it would damage the Western claim to the moral high ground in this conflict.

“Nothing would rock the credibility of Europeans and Americans more than a confirmation of the news that American mercenaries are working for the Ukrainian government. For Moscow, this would be a priceless propaganda victory where the West would appear morally naked. As naked as Moscow is now.”

Lindsay Wise contributed to this report from Washington.