The Pentagon confirmed Thursday that Chinese nationals fired lasers near a military base in east Africa against U.S. military aircraft in the region, injuring several pilots.

Pentagon Press Secretary Dana White said the U.S. government made diplomatic protests to the Chinese government over several recent incidents of laser firings near China's first overseas military base at Djibouti.

"These are very serious incidents. There have been two minor injuries. This activity poses a threat to our airmen," White told reporters.

"We have formally demarched the Chinese government, and we've requested that the Chinese investigate these incidents," she added.

The number of incidents is "more than two but less than ten" and the laser firings took place in recent weeks, White said.

White said the Pentagon is confident that Chinese nationals were behind the laser firings but did not elaborate on the intelligence linking Beijing to the incidents.

She declined to speculate on the Chinese motive behind the laser attacks.

"I believe there have been cases where this has happened previously," White said. "But what this started was these last few weeks we decided to become very serious about it, and we have demarched the Chinese, and we've asked for the investigation."

China's government has not commented on the incidents. A Chinese military expert told the state-run Global Times newspaper that the U.S. accusations that China used a laser weapon in Djibouti was "groundless."

China opened the military base in Djibouti last year and plans to deploy some 400 troops there.

China's government has asserted that the base is merely a logistics hub for anti-piracy operations as well as to support China's international infrastructure project called Belt and Road Initiative.

The U.S. government regards the Chinese base as part of Beijing's efforts to project military power around the world.

The Djibouti incidents appear similar to the 1997 incident involving a Russian merchant ship that was shadowing a Navy missile submarine and fired a laser on a Canadian surveillance helicopter.

The laser damaged the eyes of the Navy Lt. Jack Daly and Canadian pilot Captain Patrick Barnes near Washington state and was covered up by the administration of President Bill Clinton.

China and the United States have battled over international trade and finance after the Trump administration announced it will impose tariffs on China for its unfair trade practices.

The U.S. and Chinese militaries also have squared off in the South China Sea where China is seeking to take control of the strategic waterway. The U.S. military has been seeking to counter the illegal claim by sending ships and aircraft near disputed islands in the sea that have been militarized in recent years by China.

A Federal Aviation Administration notice to airmen reported April 14 that "there have been multiple lazing events involving a high power laser" near the Chinese military base.

"Use extreme caution when transiting near this area," the notice states. "If a laser is seen in or near Djibouti, notify immediately tower…"

The notice also said U.S. military air crews were to contact military air controllers.

In one incident, air crew members flying aboard a C-130 transport suffered two minor eye injuries after exposure to what was described as "military-grade laser beams" that appear to have been fired from the Chinese base.

The laser incidents followed U.S. military exercises last month off the coast of Djibouti called Alligator Dagger. The exercise was canceled April 5 after two separate air mishaps, including the crash of a Marine Corps Harrier jump jet and a CH-53 helicopters, in Djibouti.

About 4,000 U.S. personnel are stationed at Camp Lemonnier in Djibouti. The base has been used as a major launch point for U.S. military and special operations commando raids in the region.

The Chinese military base in the country is located about a mile from Camp Lemonnier and is Beijing's first overseas military base and has raised security concerns because it is located very close to the U.S. military base there.

The Washington Free Beacon disclosed in 2015 that China's military has deployed several types of hand-held blinding laser weapons.

The weapons, according to state-run Chinese media, are used to interfere and damage laser and night vision equipment.

Use of the weapons violates China's announced commitment to a section of the 1998 U.N. Convention on Certain Conventional Weapons that bans the use of blinding lasers in combat.

The Chinese have marketed four types of laser guns, designated the BBQ-905 Laser Dazzler Weapon, the WJG-2002 Laser Gun, the PY132A Blinding Laser Weapon, and the PY131A Blinding Laser Weapon.

Marine Corps Gen. Thomas Waldhauser, commander of the U.S. Africa Command, warned in congressional testimony in March that China is seeking to expand and possibly take over the strategic port in Djibouti, which owes Beijing an estimated $1.2 billion in construction debt.

China operates a naval port and "multi-purpose" port that offloads containers from freighters.

Waldhauser said a Chinese takeover of ports in Djibouti would have "significant consequences" for U.S. military operations there.

"We are not naive to think that some of the activities the Chinese are doing in terms of counterintelligence there are taking place, but it just means that we have to be cautious," Waldhauser told the House Armed Services Committee March 6. "We have to be on guard for that type of situation."