Ritchie B. Tongo, Associated Press
In this Monday, May 11, 2015, file photo, This aerial photo taken through a glass window of a military plane shows China's alleged on-going reclamation of Mischief Reef in the Spratly Islands in the South China Sea. The dispute over the strategic waterways of the South China Sea has intensified, pitting a rising China against its smaller and militarily weaker neighbors who all lay claim to a string of isles, coral reefs and lagoons known as the Spratly and the Paracel islands. Only about 45 of them are occupied.

WASHINGTON — The United States is troubled by China's plans to keep building on artificial islands in the South China Sea, the top U.S. diplomat for East Asia said Thursday.

Assistant Secretary of State Danny Russel told journalists that the prospect of China militarizing those maritime outposts runs counter to the goal of reducing regional tensions.

China has territorial disputes with several neighbors in the South China Sea. According to the U.S., China has reclaimed 2,000 acres (800 hectares) on reefs and atolls there since last year, a move that has sharpened its differences with the U.S., which says it has an interest in freedom of navigation there.

China's Foreign Ministry announced Tuesday it will complete its land reclamation within days, in an apparent bid to reassure its neighbors and perhaps to tamp down tensions with the U.S. ahead of an annual strategic and economic dialogue in Washington next week.

But China also said also that it would follow up by building infrastructure to support activities such as maritime search and rescue and scientific research. It said apart from satisfying defense goals, the main purpose of such projects was civilian in nature and not targeted at any third parties.

"Frankly, we are concerned, and others are concerned," Russel responded. "The simple fact is that neither that statement nor that behavior contributes to reducing tensions, and reducing tensions is what we all should want."

The disputed islands lie amid some of the world's busiest shipping lanes and rich fishing grounds. China claims virtually the entire South China Sea, while Vietnam, Malaysia, Brunei, the Philippines and Taiwan also say they own parts or all of it.

Russel said it would be discussed next week. The strategic track in the talks that will be led by Secretary of State John Kerry and China's State Councilor Yang Jiechi. The two sides will also grapple with thorny issues like cybersecurity and human rights in China, but seek to forge more cooperation on climate change.

"We don't always see eye to eye, but the fact is global challenges require us to cooperate," Russel said. "We work through our differences and seek to solve problems and manage the problems we can't seem to solve."