But Russian military officials said the presence of their troops there was a mistake. From Britain’s Guardian:

“The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained,” a source in Russia’s defence ministry told the RIA Novosti agency.

Ukrainian officials, meanwhile, suspect the Russian soldiers were carrying out a deliberate mission on foreign soil.