Sanctions against Russia are helping keep the Russian side at the negotiating table, Ukrainian President Petro Poroshenko said.

"I would like to say that sanctions are not a goal in themselves, but they are only a tool to ensure Russia's presence at the negotiation table and to ensure full implementation of the Minsk agreements," Poroshenko told reporters after talks with U.S. Defense Secretary James Mattis in Kyiv on Thursday.

The United States will continue putting pressure on Russia using sanctions, Mattis said.

The U.S. and its allies will continue supporting Ukraine in its struggle, and sanctions will remain until Russia begins fulfilling the earlier stated agreements, he said. The U.S. is making considerable diplomatic efforts to settle the conflict, Mattis said.