MANILA—The U.S. will start stationing warplanes in the Philippines this week as the vanguard of a major deployment to the Southeast Asian country as Washington and its allies mount a coordinated response to Beijing’s assertiveness in the South China Sea.
The U.S. and the Philippines began joint patrols of the South China Sea last month, Defense Secretary Ash Carter said Thursday on a visit to the Philippines, a U.S. treaty ally that is among the five governments whose territorial claims overlap with China’s in those waters.
Tensions have been escalating as a United Nations-backed arbitration panel in The Hague prepares to rule in a case brought by the Philippines against China’s maritime claims.
Mr. Carter’s announcements came after he scrubbed a planned visit to Beijing as part of his Asian tour amid rising U.S. concerns over China’s construction of artificial islands in the South China Sea and its recent deployment of weaponry on another disputed island. Defense officials cited scheduling difficulties as the official reason for canceling.
They also came after China summoned diplomatic envoys from the Group of Seven nations to protest over a statement they issued at a meeting in Japan this week opposing “coercive or provocative” action in the South China Sea and East China Sea.
U.S. allies have been bolstering defense ties with the Philippines, with Australia sending troops to participate in joint exercises with U.S. and Philippine forces, and Japan sending a submarine and two destroyers to visit a Philippine naval base this month.
Australian Prime Minister Malcolm Turnbull planned to warn Chinese leaders during a visit to Beijing this week that their recent muscular posture in Asia risks harming China’s economy and international relations, according people familiar with his plans. At a media briefing in Beijing on Friday, Mr. Turnbull took a circumspect tone, urging all South China Sea claimants to settle disputes peacefully; He declined to comment on the U.S. deployment.
China claims sovereignty over all South China Sea islands and their adjacent waters and has pledged not to “militarize” the structures it has built. But U.S. officials say it has recently deployed fighter jets and surface-to-air missiles on another disputed island to the north.
There is also concern in Washington and Manila that China, having almost completed seven artificial islands in the disputed Spratlys archipelago, is planning to build a military outpost at a disputed reef less than 200 nautical miles from Manila.
“In the South China Sea, China’s actions…are causing anxiety and raising regional tensions,” Mr. Carter told reporters at the Philippines’ presidential palace, where he met President Benigno Aquino III. The U.S. deployment is designed “to tamp down tensions here” and wouldn’t provoke a showdown with Beijing, he said.
China’s Defense Ministry strenuously objected, saying the latest U.S.-Philippines military cooperation would exacerbate tensions. It said the joint-patrol plan “promotes the militarization of the region” and called the strengthened military alliance and joint exercises “the embodiment of Cold War thinking and not conducive to peace and stability in the South China Sea.”
The U.S. and the Philippines have been holding 10 days of joint drills that end Friday. Mr. Carter said five American A-10 Thunderbolt ground-attack jets, three H-60G Pavehawk helicopters and one MC-130H Combat Talon special forces infiltration aircraft will remain behind at Clark Air Base north of Manila along with 200 crew members.
Philippines Defense Secretary Voltaire Gazmin said he hoped the U.S. moves would “deter uncalled-for actions by the Chinese.”
Last month, the Philippines said it would make five military bases available to U.S. forces under the terms of a new defense pact signed in 2014. Earlier in the week, Mr. Carter visited India, which is also upgrading its security ties with Washington.
Since the start of the U.S. “pivot” to Asia early in the Obama administration, the Pentagon has moved to beef up its presence in the region to counter China’s rising military power, including with additional personnel, ships and aircraft. U.S. officials have said that by 2020, 60% of the Navy’s ships and aircraft will be deployed to the Pacific, up from about half before the rebalance.
“We’re sending our most advanced warfighting platforms to the region, including multi-mission ballistic missile defense-capable ships, submarines, and intelligence, surveillance and reconnaissance aircraft,” a Navy spokesman said.
The Air Force has a large presence across the Pacific. That includes a program in which it has a “continuous bomber presence” that aims to demonstrate American commitment to the region. Pacific Air Forces conducted about 100 continuous bomber presence and bomber assurance and deterrence missions across the Western Pacific in 2015, according to the Air Force. Currently, B-52 bombers fly those missions from Andersen Air Force Base, Guam. “Everything we’re doing out here is sending a message to our allies and partners of our commitment to regional security,” said Lt. Col. Damien Pickart, a spokesman for U.S. Pacific Air Forces, in Honolulu.
Dismay about China’s plans deepened last month when U.S. Navy Chief Adm. John Richardson said the U.S. was monitoring increased Chinese activity at Scarborough Shoal, a disputed cluster of reefs, rocks and sandbars that China has controlled since a standoff with the Philippines in 2012. The activity might signal Beijing’s intention to build a military outpost there, he said, on what China considers its sovereign territory.
China has reclaimed land and built facilities at seven other reefs in the South China Sea, ignoring the objections of neighbors. China’s maritime claims cover most of the sea, and overlap with those of Malaysia, the Philippines, Vietnam, Brunei and Taiwan.
Scarborough Shoal is much closer to the Philippines than the seven islets China has been building so far, making any Chinese military outpost there a more overt threat.
The U.S. last year resumed performing what it termed freedom-of-navigation operations in the South China Sea to challenge China’s claims. But those patrols won’t count for much if the U.S. fails to prevent China from fortifying Scarborough Shoal, said Gregory Poling of the Center for International and Strategic Studies, a U.S. think tank.
In recent days, the Obama administration has faced calls from Sen. John McCain (R., Az.) and others to offer explicit guarantees to defend the Philippines in the face of Chinese assertiveness.
“The pressure is building up from the Senate and within the Pentagon towards an explicit American guarantee that Scarborough Shoal falls within the Philippine-U.S. mutual defense treaty, thus providing a pretext for a more robust American pushback,” said Richard Javad Heydarian, a regional-security specialist at Manila’s De La Salle University.
—Gordon Lubold in Washington contributed to this article.
Write to Trefor Moss at Trefor.Moss@wsj.com