(Photo by Petty Officer 3rd Class David Cox)

The US Navy and regional allies have reportedly noticed an increase in Chinese radio queries to foreign ships and planes operating in the South China Sea — some said to be less than friendly, and others actually threatening.

"Leave immediately," Chinese forces in the disputed Spratly Islands warned in early 2018 when a Philippine military aircraft flew close to a Chinese outpost, The Associated Press reported July 31, 2018, citing a new Philippine government report.


"Philippine military aircraft, I am warning you again, leave immediately or you will pay the possible consequences," the report said the Chinese forces threatened soon after, according to the AP.

In the latter half of 2017, Philippine military aircraft patrolling near contested territories received at least 46 Chinese radio warnings, the government report says, according to the AP. While these warnings have traditionally been delivered by Chinese coast guard units, they're now thought to be broadcast by personnel stationed at military outposts in the South China Sea, the news agency reported.

US Navy destroyers.

(US Navy photo)

"Our ships and aircraft have observed an increase in radio queries that appear to originate from new land-based facilities in the South China Sea," Cmdr. Clay Doss, a representative for the US 7th Fleet, told the AP.

"These communications do not affect our operations," he added, noting that when communications with foreign militaries are unprofessional, "those issues are addressed by appropriate diplomatic and military channels."

The Philippine military tends to carry on with its activities. "They do that because of their claim to that area, and we have a standard response and proceed with what we're doing," Philippine air force chief Lt. Gen. Galileo Gerard Rio Kintanar Jr. told the AP.

Though an international arbitration tribunal sought to discredit China's claims to the South China Sea two years ago, China has continued to strengthen its position in the flashpoint region.

In recent months, China has deployed various defense systems — such as jamming technology, surface-to-air missiles, anti-ship ballistic missiles, and even heavy bombers — to the South China Sea, leading US Secretary of Defense Jim Mattis in June 2018 to accuse China of "intimidation and coercion" in the waterway.

An EA-18G Growler takes off from a flight deck .

(U.S. Navy photo by Mass Communication Specialist Seaman Ethan J. Soto)

Beijing, however, argues that it has a right to defend its sovereign territory, especially considering the increased frequency of US Navy freedom-of-navigation operations in the area; it has conducted more than half a dozen since the start of the Trump administration.

Despite Chinese warnings and objections, the US military has repeatedly made clear that it will maintain an active military presence in the South China Sea.

"International law allows us to operate here, allows us to fly here, allows us to train here, allows us to sail here, and that's what we're doing, and we're going to continue to do that," Lt. Cmdr. Tim Hawkins told the AP in February 2018.

The US military has also expressed confidence in its ability to deal with China's military outposts in the region should the situation escalate.

"The United States military has had a lot of experience in the Western Pacific taking down small islands," Lt. Gen. Kenneth McKenzie, the director of the Joint Staff, told reporters in Ma 2018, adding: "It's just a fact."

In early 2018 the US disinvited China's People's Liberation Army Navy from participating in 2018's iteration of the multilateral Rim of the Pacific maritime exercises, citing what it characterized as alarming Chinese activities in the South China Sea. The Philippines has at least twice raised the issue of radio warnings with Beijing, the AP reported July 31, 2018.

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.