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(CNN) —  

Chinese personnel at the country’s first overseas military base in Djibouti have been using lasers to interfere with US military aircraft at a nearby American base, activity that has resulted in injuries to US pilots and prompted the US to launch a formal diplomatic protest with Beijing, two military officials told CNN.

The US issued a notice to airmen “to exercise caution when flying in certain areas in Djibouti,” which “was issued due to lasers being directed at US aircraft on a small number of separate occasions over the last few weeks,” according to the notice obtained by CNN.

“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,” the notice said.

Two US military officials told CNN that the issue was of major concern as such activity can cause major accidents.

Military-grade laser beams, occasionally known as “dazzlers,” omit a powerful beam of light that can travel great distances and be used to illuminate aircraft cockpits, temporarily blinding pilots.

The officials said that the State Department had lodged a formal diplomatic protest with Beijing in an effort to get China to stop the activity.

Chief Pentagon spokeswoman Dana White confirmed the incidents later on Thursday, saying the United States has “formally demarched” the Chinese government over the incidents and has “requested” that the Chinese launch their own investigation of the situation.

“This activity poses a true threat to our airmen,” White said, later saying that the incidents had grown increasingly serious over the last few weeks.

A US defense official told CNN that the US military also believes the Chinese use similar lasers to interfere with US aircraft in the South China Sea.

A 2015 report in the official Chinese military newspaper the PLA Daily noted that “China has been updating its home-made blinding laser weapons in recent years to meet the needs of different combat operations.”

According to the report, Chinese forces have access to at least four different types of portable blinding laser weapons, all of which look like oversized modified assault rifles.

Both China and the US are signatories to the Protocol on Blinding Laser Weapons, which prohibits the use of blinding laser weapons as a means or method of warfare.

Competing military bases

Last month the US military was forced to briefly halt air operations in the East African nation of Djibouti, a critical location in the fight against terrorism, following a series of accidents involving aircraft. The halting of air operations was done at the request of Djiboutian government.

Chinese Foreign Ministry spokeswoman Hua Chunying said in her daily briefing Friday the US had been told their accusations were “totally inconsistent with facts.” “You can remind people in the US they should pay attention to facts and not make groundless accusations,” she said.

On Thursday, Chinese state media had strongly refuted the original claims by US defense officials, accusing them of “cooking up phony laser stories.”

Quoting Chinese military experts, state media tabloid Global Times said Beijing’s Djibouti base was “small and serves as logistical support.”

“The US should treat the base with a fair attitude and stop making up rumors about it,” the expert was quoted as saying.

There are about 4,000 US personnel in Djibouti, based at Camp Lemonnier. The US military places a lot of importance on its ability to base forces in Djibouti given its critically strategic location near countries like Somalia and Yemen, where the US regularly targets terrorists in airstrikes.

But US officials have recently expressed concern about the growing influence of China in Djibouti, noting its establishment of its first military base there and its close economic links with the country.

Djibouti relies heavily on investment capital from the Chinese government and the two have close diplomatic ties.

In late February, US lawmakers raised concerns Djibouti was planning to hand over control of a major commercial port to China, after the local government seized the port and canceled the previous operator’s contract.

The port, located beside the Chinese military base, is the primary access point for American, French, Italian and Japanese bases in Djibouti.

Gen. Thomas Waldhauser, who oversees US Africa Command, acknowledged both challenges during an appearance before Congress in March.

“We are taking significant steps on the counterintelligence side so that we have all the defenses that we need there, there is no doubt about that,” said Waldhauser, referring to the proximity of the new Chinese base.

CNN’s Steve George and Brad Lendon contributed to this report.