THE United States Navy has stepped up its pressure on China over its island building programme in the Spratly Islands by releasing video footage and audio recordings of exchanges with Chinese naval ships, reports IHS Jane's Defence Weekly.

The US Navy also allowed CNN television news on a Boeing P-8 Poseidon surveillance flight from Clark Air Force Base in the Philippines to the Spratlys. 

During the flight the P-8 was warned eight times by the People's Liberation Army Navy (PLAN) that it was approaching "a military alert zone" and that it should leave the area "to avoid misjudgement".

On audio recordings released by the US Pacific Fleet, the PLAN can be heard saying, "This is the Chinese Navy, this is the Chinese Navy, this is Chinese sky.

"Foreign military aircraft, this is the Chinese navy, you are approaching our military alert zone. Please leave immediately to avoid misjudgement."

A US Navy officer repeatedly replied: "I am a United States military aircraft conducting lawful military activities outside national airspace. I am operating with due regard, as required, under international law."

Speaking in Washington, Daniel Russel, assistant secretary of state for East Asian and Pacific affairs, said the US would continue such flights. 

"We will continue to fully exercise our rights globally to the international space," he said. "Nobody in their right mind is going to try to stop the US Navy from operating."

Meanwhile, a Pentagon spokesman suggested that the P-8 had not entered the 12 nautical mile territorial zone around the reefs. 

US officials told the Wall Street Journal in mid-May that the Pentagon was considering air and surface fleet activities within the 12 nautical mile zone as part of "freedom of navigation" patrols.

Chinese Foreign Ministry spokesman Hong Lei condemned the P-8 flight, describing it as "completely irresponsible and completely dangerous".

"Such action is likely to cause an accident; it is completely irresponsible and completely dangerous and detrimental to regional peace and stability," he said. 

"We express our strong dissatisfaction, we urge the US to strictly abide by international law and international rules and refrain from taking any risky and provocative actions," he said.

The absence of significant building construction at Fiery Cross so far contrasts with other, small sites in the Spratlys such as Hughes Reef. 

Vietnamese media published photographs showing a nine-storey structure on the newly built island at Hughes Reef, only a few kilometres from the Vietnamese-occupied Sin Cowe Reef.