The U.S. has issued a formal complaint to the Chinese government over the use of high-grade lasers near the military base in Djibouti that were directed at aircraft and resulted in minor injuries to two American pilots, the Pentagon said Thursday.

Pentagon spokeswoman Dana White said that the U.S. is confident that Chinese nationals are responsible for the use of the lasers, which targeted aircraft on several occasions in the last few weeks. White said the incidents represent a serious threat to U.S. airmen, and that the U.S. has asked China to investigate the incidents.

She estimated there have been fewer than 10 laser incidents in recent weeks, and the Pentagon sought the formal complaint because of the injuries and the growing number of instances the problem happened.

Marine Lt. Col. Chris Logan, a Pentagon spokesman, said that reports from pilots indicate that on three occasions the lasers were military grade and came from the Chinese base nearby.

The Chinese recently built a base near Camp Lemonnier, the U.S. military installation in the Horn of African nation of Djibouti. It is China's first overseas military base, and was manned last year. It is just a few miles from the U.S. base.

In addition to issuing the formal diplomatic demarche, the U.S. also put out a notice to airmen, saying they should exercise caution when flying near certain areas in Djibouti.

The pilots suffered minor eye injuries. There were no aircraft crashes or other more serious problems.

Lasers present a serious problem because when aimed at aircraft they can injure pilots or temporarily blind them -- which can present safety risks particularly as they are taking off and landing.