Kirk Spitzer, USA TODAY

Crew members wait for clearance to launch an F/A-18 Super Hornet from the deck of the USS John C. Stennis in the Philippine Sea Wednesday during joint naval exercises with the Japanese Maritime Self-Defense Force and the Indian Navy.(Photo: Kirk Spitzer, USA TODAY)

ABOARD THE USS JOHN C. STENNIS —

When U.S. commanders kicked off a major international naval exercise this week, they found an uninvited guest: A Chinese Navy warship.

But it was no surprise.

Warships from the People’s Liberation Army Navy (PLAN) have begun to routinely shadow U.S. Navy ships through much of the region.

U.S. commanders said Wednesday Chinese warships closely followed the powerful USS John C. Stennis carrier strike group from nearly the minute it entered the disputed South China Sea on a regular patrol in early March.

“We did see PLAN ships quite routinely throughout the South China Sea. In fact, we were in constant visual contact with at least one PLAN ship at any one time, 24/7,” said Rear Adm. Marcus Hitchcock, commander of the Stennis strike group.

That’s a major departure from recent years and seems to provide further evidence of China’s increasingly assertive behavior in the region.

Twice in the last week, PLAN warships have sailed into or alongside Japanese territorial waters, including those around the disputed Senkaku Islands, in the East China Sea. That’s sparked protests from Japan, one of America’s closest allies.

Warships from the U.S., Japan and India kicked off the sea portion of a weeklong joint exercise Wednesday in the Philippine Sea, not far from Japan’s southernmost islands.

The Navy describes the “Malabar” war games as “complex, high-end, war-fighting exercises” designed to increase the ability of U.S., Indian and Japanese naval forces to operate together.

Malabar began in 1992 as a bilateral exercise between India and the U.S. but has expanded in recent years to include Japan. This is the second time in three years that the exercises have been held near Japanese waters and marks an important shift in Indian relations toward the U.S. and its allies.

India is embroiled in a long-running border dispute with China and is wary of the PLAN’s increasing operations in the Indian Ocean.

Last week, Indian Prime Minister Narendra Modi told a joint session of the U.S. Congress that a strong India-U.S. partnership “can anchor peace, prosperity and stability from Asia to Africa and from the Indian Ocean to the Pacific.”

The nuclear-powered Stennis is nearing the end of a planned six-month patrol through the Asia-Pacific region. Its strike group includes four other surface warships and a Los Angeles-class attack submarine.

Hitchcock said Wednesday that PLAN warships had followed as close as three to four miles from the USS Stennis while it was operating in international waters in the South China Sea.

He said a PLAN intelligence gathering ship had followed the Stennis group into the Philippine Sea last week. It was operating about seven to 10 miles from the carrier on Wednesday during the Malabar exercises, which are taking place in international waters.

Stennis’ commanding officer, Capt. Gregory Huffman, said the Chinese ships have operated in a professional manner throughout and that the Stennis group has been able to conduct operations unimpeded.

That’s a good thing if it lasts, said Jeffrey Hornung, an East Asia security specialist with the Sasakawa Peace Foundation USA, in Washington, D.C.

“The behavior the PLAN is exhibiting is not behavior that the U.S. Navy would engage in, nor is it behavior that we would expect other rule-of-law abiding nations to conduct. But it has come to be the norm for China,” Hornung said.

China claims nearly all of the South China Sea and has used massive dredging operations to build at least seven artificial islands, some with military-grade runways and port facilities.

More than $5 trillion in annual trade passes through the waterway and U.S. and officials are concerned that China could use the islands to restrict air and sea navigation. China has said it won’t do that.

U.S. Defense Secretary Ashton Carter made a high-profile visit to the Stennis while it was in the South China Sea in April to criticize China’s territorial claims and island-building campaign.

Taiwan and four other countries also claim parts of the South China Sea. An international tribunal based in The Hague is expected to rule soon on a legal challenge by the Philippines against China’s claims in the region; China has said the tribunal has no authority and that it will ignore the ruling.

Everyone aboard the Stennis is aware of the Chinese ships shadowing the fleet, Lt. James Brigden, a launch and recovery officer, said.

“It’s always interesting to see our Chinese companion ‘in formation’ with the other ships. Their presence is always felt,” Brigden said. "We just try to treat it like another day, keep everybody focused on their jobs.”