Israel's air force says it has found cancerous material on additional warplanes.

The air force last month grounded its fleet of F-16i fighter plans after finding dangerous levels of formaldehyde on an aircraft. The army says it has now tested about half its fleet, and that about 30 percent of the warplanes tested so far had excessive amounts of formaldehyde.

The army does not say how many planes it has in its fleet.

Israel's air force chief ordered a halt to training on the F-16i planes on March 21 after pilots complained about odors in the cockpit. Subsequent checks revealed that formaldehyde had leaked into the cockpit in some of the planes.

The army says it will investigate the matter with the plane's maker, U.S. company Lockheed Martin Corp.