Read alsoU.S. pushes NATO to ready more forces to deter Russian threat

"Successful implementation of defense reform in Ukraine is directly connected with our common security – U.S. and NATO nations security," Mattis said during a meeting of Ukrainian Defense Minister Stepan Poltorak with U.S., UK, Canada, Lithuania, and Poland defense ministers in Brussels, the press service of Ukraine's Defense Ministry has reported.

Mattis praised the great contribution of Ukrainians to defend the territorial integrity and independence of Ukraine.

Moreover, talks also dealt with the recent decision to provide Ukraine with a new tranche of the U.S. assistance, which will permit to enhance dynamics and content of the ongoing large-scale reform within the Defense Ministry of Ukraine.

"Russia violates the international norms and rights. We have to proceed with high level meetings and assist Ukraine," Mattis said.