The United States has issued a formal warning to China after personnel at the Chinese military base in Djibouti used lasers to interfere with U.S. military aircraft, giving two pilots minor injuries, according to the Pentagon.

Top Defense Department spokeswoman Dana White told reporters Thursday that the U.S. is confident the Chinese are behind the “very serious incidents,” which have increased in the past few weeks.

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“There have been two minor injuries. This activity poses a true threat to our airmen. We have formally demarched the Chinese government. And we’ve requested that the Chinese investigate these incidents,” White said during a Pentagon briefing.

The U.S. government, meanwhile, has warned airmen to be cautious when flying in certain areas in Djibouti, on the Horn of Africa, due to the recent incidents.

“During one incident, there were two minor eye injuries of aircrew flying in a C-130 that resulted from exposure to military-grade laser beams, which were reported to have originated from the nearby Chinese base,” according to a notice reported by CNN.

White said there have been “more than two [and] less than 10” such instances where the Chinese used a high-power laser to disrupt pilots. The incidents have happened in the past but they have picked up in the past few weeks.

“It’s enough that we’re concerned. ... It’s a serious matter and so we’re taking it very seriously,” she said.

The Pentagon has about 4,000 personnel based at Camp Lemonnier in Djibouti, the only permanent U.S. military base in Africa. The U.S. uses the base as a launching point for airstrikes in Somalia and Yemen.

The Chinese base — its first overseas military location — is several miles north of Camp Lemonnier.

Last month, a Marine AV-8B Harrier jet crashed at Djibouti Ambouli International Airport, with the pilot ejecting. Hours later, a Marine CH-53 Super Stallion helicopter suffered structural damage during a landing.

Both incidents happened during Alligator Dagger, an annual amphibious exercise off the coast of Djibouti, and are under investigation.