According to Paul Picard, the chief OSCE observer in eastern Ukraine, Russian vehicles were seen moving between Russia and Ukraine in both directions. Twenty-one vehicles were labeled “Cargo-200,” a Russian military code for the bodybags of soldiers killed in action.

At a press conference, Picard also reported that, between September 1, 2014, and August 4, 2015, observers saw 21,309 individuals in military uniform crossing the border at the “Gukovo” and “Donetsk” checkpoints. Picard says these men identified themselves as “volunteers,” and that the movement of uniformed men across the border has continued in recent days.

Observers report that they did not see any military equipment crossing the border.

Correction: an earlier version of this story claimed that the OSCE observed 21,309 volunteers entering Ukraine between August 1, 2015, and August 4, 2015. We apologize for the error.

Earlier, Paul Picard told The New York Times that the border with Ukraine at these checkpoints “is porous” and allows Russian military convoys to cross into Ukraine without interference. The New York Times reported that an average of 500 people who appear to be soldiers have crossed the border every week. Picard said one of the men reported: “They told us they cannot cross with weapons, but they receive weapons on the other side.”

In May, Reuters reported that there has been a dramatic increase in the number of Russian soldiers deployed at military bases in the region bordering Ukraine. Reuters reported that some of the soldiers are wearing uniforms without insignia.

Moscow has systematically denied that any Russian soldiers on active duty are participating in the conflict in Ukraine.