U.S. Defense Secretary Robert Gates on Monday said he was encouraged by Lockheed Martin Corp's efforts to address issues with the F-35 fighter, and said company officials clearly understood the importance of keeping the program on cost and on schedule.

Gates, speaking to reporters during a tour of the plant where Lockheed builds the F-35, said officials were working on improvements in the manufacturing process and the supply chain for the fighter.

He said he could not guarantee that problems would not arise in the future, but he remained convinced that the plan to build three variants of the new fighter -- for a total of more than 3,000 airplanes -- would save money in the long run. We cannot afford, as a nation, not to have this airplane, he said. (Reporting by Andrea Shalal-Esa, editing by Maureen Bavdek)