Representatives of the separatist self-proclaimed Donetsk People's Republic inspect a burned tank during a joint operation to search for the remains of killed soldiers at the city's airport in Donetsk, Ukraine, May 22, 2015. Reuters

The Defense Ministry of the self-proclaimed Donetsk People’s Republic (DPR) claimed Friday that Kiev forces were preparing to use chemical weapons against the pro-Russia rebels. The ministry’s official spokesman, Eduard Basurin, said Ukrainian Armed Forces had begun in recent weeks to strengthen security within its facilities and dismiss civilian workers, reported Tass, a Russian state news agency.

"A week ago, our intelligence saw that 20 truck tanks with a capacity of five tons with an unknown chemical agent arrived at the facility’s territory," Basurin said. Basurin also claimed a group of U.S. military chemical experts had arrived in the southern port city Mariupol on Tuesday to assist with the campaign. He predicted that "U.S. military experts and Kiev authorities will prepare sabotage on Ukraine’s territory with the aim of accusing the DPR leadership of crime against humanity."

Russian media has alleged in the past that Ukraine was directing chemical weapons at separatist rebels. In August 2014, RIA Novosti reported that Ukrainian forces had used chemical weapons in Donetsk, including a gas that affected "sense organs." Before that, eastern Ukrainian rebels claimed in June 2014 that the army had used an unknown chemical weapon in the settlement of Semyonovka near the city of Slavyansk.

This week, Russian President Vladimir Putin banned the release of information about troop deaths in "special operations" during peacetime amid continued denials by the Kremlin that its soldiers are fighting with the rebels in Ukraine. Under the law, anyone who reveals details about the deaths of soldiers sent on operations could now be prosecuted.

"We see this as a misplaced effort to cover up what everyone knows, and that is that Russian active-duty military personnel are fighting and dying in eastern Ukraine and that the Russian government is denying it," U.S. State Department spokesman Jeff Rathke said.

More than 6,000 people have been killed in eastern Ukraine since April 2014 "in spite of successive ceasefires," the United Nations Human Rights Office has said.