“Anyone with no invasive intention will find no reason to worry about this,” she added.

While China has long claimed the islands, reefs and other outcroppings within the South China Sea, other nations also have claims there, including Vietnam and the Philippines.

China’s vast reclamation project, which began in earnest in 2013, shortly after Mr. Xi became the country’s paramount leader, has steadily turned once-uninhabited places into fortified islands with airfields and increasingly military outposts. In doing so it has brushed aside warnings from the United States and other nations and even a ruling against its territorial claims by an international arbitration panel in 2016.

China’s base in Djibouti, its first overseas, has long been a source of concern for the United States and other militaries operating around the Horn of Africa. It opened last year and has been portrayed by the Chinese as a logistics base to support antipiracy, counterterrorism and humanitarian operations in Africa and the Middle East.

It also happens to be just a few miles from the only permanent American base in Africa, which was established after the Sept. 11, 2001, terrorist attacks. The base, operated by the Navy adjacent to Djibouti’s international airport, is home to some 4,000 personnel, including those involved in highly secretive missions in the region, including at least two Navy SEAL raids into Yemen.

The use of lasers was first made public in April in a warning to pilots issued by the Federal Aviation Administration. It noted that there had been multiple instances of “a high-power laser” being used near where the Chinese base is. Using lasers to disorient or disable pilots is an old military tactic, but an international protocol adopted in 1995 and joined by China prohibits the practice.

Ms. White said that there was no doubt about the origin of the lasers, and that the Pentagon had asked the Chinese to investigate. “It’s a serious matter,” she said, “and so we’re taking it very seriously.”

In a statement on Friday afternoon, China’s Ministry of National Defense strongly disputed the Pentagon’s accusations, saying they were “completely inconsistent with fact.”