"The Ukrainian Security Service (SBU) opened criminal proceedings against the head of the Russian Border Service," said SBU chief Valentin Nalivaichenko, Xinhua reported citing Interfax news agency.
Nalivaichenko said the SBU detained Russian agents, including those working for the Federal Security Service (FSB), on charges of financing and supporting insurgents in Ukraine, as well as arms smuggling to the rebellious Donetsk and Lugansk regions.
The Russian Border Service is a subdivision of the FSB, the successor to the former Soviet Union's State Security Committee (KGB).
Russia said one of its border checkpoints in south Rostov region was hit Thursday by mortar shells from Ukraine.
Moreover, Russian foreign ministry Friday denied accusations by the British foreign office on Russia supplying Ukrainian rebels with arms, calling these allegations "shameless".
In a phone conversation later on Friday, German Chancellor Angela Merkel, French President Francois Hollande and Ukrainian President Petro Poroshenko agreed that a common approach by Ukraine and Russia to secure their border is one of the preconditions to stabilise the situation in Ukraine.