WASHINGTON (REUTERS) - China is building some "fairly amazing submarines" and now has more diesel and nuclear-powered vessels than the United States, a top US Navy admiral told lawmakers on Wednesday, although he said their quality was inferior.

Vice-Admiral Joseph Mulloy, deputy chief of naval operations for capabilities and resources, told the House Armed Services Committee's seapower sub-committee that China was also expanding the geographic areas of operation for its submarines, and their length of deployment.

For instance, China had carried out three deployments in the Indian Ocean, and had kept vessels out at sea for 95 days, Vice-Admiral Mulloy said.

"We know they are out experimenting and looking at operating and clearly want to be in this world of advanced submarines," Vice-Admiral Mulloy told the committee.

US military officials in recent months have grown increasingly vocal about China's military build-up and launched a major push to ensure that US military technology stays ahead of rapid advances by China and Russia.

Vice-Admiral Mulloy said the quality of China's submarines was lower than those built by the United States, but the size of its undersea fleet had now surpassed that of the US fleet. A spokesman said the US Navy had 71 commissioned US submarines.

US submarines are built by Huntington Ingalls Industries and General Dynamics Corp.

In its last annual report to Congress about China's military and security developments, the Pentagon said China had 77 principal surface combatant ships, more than 60 submarines, 55 large and medium amphibious ships, and about 85 missile-equipped small combatants.

Vice-Admiral Mulloy did not provide details about the number of surface ships now operated by China.

He said the US military did not believe China carried nuclear missiles on its submarines, but that it had been producing missiles and testing them.