The United States is spying over the South China Sea by one of its newest spy planes and with the presence of US Pacific Fleet Commander Adm. Scott Swift.

The US Pacific Fleet announced on Sunday that the admiral joined the seven-hour surveillance mission on Saturday.

It added that Adm. Swift witnessed the full range of capabilities of the P-8A Poseidon plane during the spying flight, the Associated Press reported.

The admiral "was pleased with the capabilities of the Poseidon," said US Navy Capt. Charlie Brown, a Pacific Fleet public affairs officer who flew with Swift on board the aircraft.

However, he did not give further details about the location of the mission.

Washington is stepping up its pressure on China after Beijing decided to build artificial islands in the South China Sea.

China has been building artificial islands in parts of the South China Sea, drawing criticism from its neighbors including the Philippines, Vietnam, Malaysia, Taiwan and Brunei.

During the past months, the US has stepped up surveillance flights over the Chinese construction projects.

The Chinese navy in May issued warnings to a US surveillance plane flying over the islands.

In a press conference in Manila, Philippines on Friday, Adm. Swift said American troops are ready to respond to any contingency in the South China Sea.

He said he was "very satisfied with the resources that I have available to me as the Pacific Fleet commander."

"We are ready and prepared to respond to any contingency that the president may suggest would be necessary," the commander added.