U.S. National Security Adviser John Bolton said during a trip to Kiev that Ukraine had made progress in its efforts to join the North Atlantic treaty Organisation, but that it still had more work to do.

Bolton said it was important to resolve the Ukraine crisis and that it would be dangerous to leave the situation as it is in Crimea and Eastern Ukraine.

Speaking to reporters after talks with president Petro Poroshenko, Bolton said Kiev should consider inviting U.S. Companies to explore Gas in Ukraine.