Lockheed Martin Corp on Tuesday said its operating margin reached 13.4 percent in the first quarter, matching an earlier record, but it would be hard to maintain that level in coming years.

Chief Financial Officer Bruce Tanner told analysts on Tuesday the company had already downsized its facilities and workforce significantly in recent years, and was continuing to work closely with suppliers to reduce costs.

He said the growing importance of the F-35 fighter jet program, which accounts for about 16 percent of overall company revenues but generates narrower margins, meant it would be hard to keep operating margins above 13 percent in coming years.

"That's a hard hurdle for us to maintain and especially in the near term," Tanner said on an earnings call. "With the significant growth coming on the F-35 program at lower than the overall margin rate, that's clearly going to put pressure on our ability to achieve that."

Lockheed's operating margin was 12.1 percent in the first quarter of 2013.

(Reporting by Andrea Shalal)