BEIJING - China on Wednesday expressed "serious concern" about reports that the U.S. is considering sending military ships and planes to challenge Chinese claims to islands it is building in the South China Sea, and said it would resolutely defend what it considered its sovereign territory.

Foreign Ministry spokeswoman Hua Chunying said the U.S. needs to clarify its stance on the matter and that countries should avoid "risky and provocative approaches to maintain the regional peace and stability."

The Wall Street Journal first reported the remarks out of Washington, citing an anonymous official as saying Defense Secretary Ash Carter had asked for ideas about how to address China's moves to reinforce islands it occupies in the strategically vital area. Plans under consideration included sending ships and aircraft to within 12 nautical miles of the built-up sites, the report said.

While the U.S. military already operates in the South China Sea, crossing the 12 nautical mile (22.2 kilometer) territorial limit around the islands could raise tensions if China chose to respond.

"We express serious concern about the U.S. remarks," Hua told reporters at a daily news briefing.

While China's upholds freedom of navigation in the area, that "doesn't mean foreign military ships and aircrafts can freely enter into another country's territorial airspace and seas. China will firmly maintain its territorial sovereignty," Hua said.

China claims virtually the entire South China Sea and its island groups, and has alarmed Washington and its neighbors by embarking on an ambitious island-building program to expand its claims, potentially for military use or airstrips, that has added about 2,000 acres of dry land.

Six governments in all claim all or part of the region, which is home to some of the world's busiest shipping lanes, along with rich fishing grounds and a potential wealth of undersea resources.

Chinese officials defend the reclamation, saying it is Beijing's territory and that the buildings and infrastructure are for public service use and to support fishermen, as well as to assert Chinese sovereignty.

It accuses the Philippines, Vietnam and others of carrying out their own building work on other islands.

The U.S. says it takes no position on sovereignty claims in the South China Sea, but has an interest in peace and stability in its busy shipping lanes.