Ukraine has sent a letter to the U.S. and EU countries asking them to increase pressure on Russia to free Ukrainian pilot and MP Nadia Savchenko, Ukrainian President Petro Poroshenko has said.

"Ukraine has sent a letter to the EU and the U.S. with a request to increase pressure on Russia to release Nadia Savchenko," Poroshenko wrote on his page on Twitter on Monday.

For his part, Poroshenko's press secretary Sviatoslav Tseholko wrote on Twitter that "on behalf of the president, the Ukrainian consul general in Rostov tomorrow will again be attempting to see Savchenko.