WASHINGTON — The United States has growing concerns that China's maritime claims in the disputed South China Sea are an effort to gain creeping control of oceans in the Asia-Pacific region, a senior U.S. official said on Wednesday.

In congressional testimony, U.S. Assistant Secretary of State for East Asian and Pacific Affairs Danny Russel said China's vague territorial claims in the South China Sea had "created uncertainty, insecurity and instability" among its neighbors.

While the United States says it does not take sides in disputes, Russel said it has an interest in seeing maritime disputes resolved peacefully. The United States has also stepped up its military presence in the region as part of a strategic "pivot" toward Asia.

"There are growing concerns that this pattern of behavior in the South China Sea reflects incremental effort by China to assert control over the area contained in the so-called 'nine-dash line' despite objections of its neighbors and despite the lack of explanation or apparent basis under international law regarding the scope of the claim itself," Russel told the House of Representatives subcommittee on Asia and the Pacific.

China has fired off a barrage of historical records - known as the nine-dash line - to depict its maritime territorial claims in the South China Sea. The nine-dash line takes in about 90 percent of the 3.5 million square kilometer (1.35 million square miles) South China Sea on Chinese maps.

China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei all have territorial claims across a waterway that provides 10 percent of the global fisheries catch and carries $5 trillion in ship-borne trade.

Russel said under international law, maritime claims in the South China Sea had to be based on land features. Beijing should "clarify or adjust its nine-dash line claim to bring it in accordance with the international law of the sea," he added.

To address U.S. concerns, Russel said he and other senior U.S. officials had traveled to the region to raise the issue with China. In prepared remarks for the testimony, he said U.S. Secretary of State John Kerry also would soon visit the region.

Russel said the United States was also concerned about the "serious downturn" in China-Japan relations over a tiny set of uninhabited islands in the East China Sea. Both Tokyo and Washington have said they do not recognize China's announcement last year that it has the right to police the skies above the islands.

Russel said the United States supported Japan's call for diplomacy and crisis management to "avoid a miscalculation or a dangerous incident."

"Neither these two important countries nor the global economy can afford an unintended clash that neither side seeks or wants," Russel said. "It is imperative that Japan and China use diplomatic means to manage this issue peacefully and set aside matters that can't be resolved at his time," he added.