China's navy is reported to have issued eight warning messages to a U.S. military surveillance plane that passed over a group of man-made islands Beijing is using to secure its sweeping territorial claims in the disputed South China Sea.

"This is the Chinese navy, this is the Chinese navy, please go away quickly," said a voice in English, according to a report by CNN, whose reporters were allowed to fly on the U.S. P8-A Poseidon spy plane Wednesday.

CNN said the U.S. Navy's most advanced spy aircraft passed as low as 4,500 meters over the disputed reefs and shoals in the Spratly Islands, which are nearly 1,000 kilometers from the shores of mainland China.

U.S. officials have expressed increasing concern about China's land-reclamation efforts at seven sites in the Spratlys, parts of which are also claimed by the Philippines, Vietnam, Malaysia, Brunei and Taiwan.

CNN footage showed extensive military buildup at some of the sites, including an early warning radar system, military barracks, lookout tower and runway that have recently been built at the Fiery Cross Reef.

'You! Go!'

When the U.S. plane refused to leave the area, a Chinese official sent another radio transmission, this time in an exasperated voice: "This is the Chinese navy. You! Go!"

A spokesman for China's foreign ministry said he was not aware of the incident, but said he hopes the "relevant country" can respect China's "indisputable sovereignty" over the area.

The surveillance flight appeared to be in line with recent comments by U.S. defense officials, who said they are considering sending military ships and planes to the South China Sea to enforce freedom of navigation.

The proposed flights, which are meant to send a message that Washington does not accept China's claims to the area, were quickly condemned as provocative by Beijing.

The U.S. military has an extensive presence in East Asia and has repeatedly flown surveillance flights close to the artificial islands. But the Wednesday mission appears to be one of the boldest challenges yet to China's control in the area.

Fears of a confrontation were also raised in late 2013, when the U.S. military flew a pair of B-52 bombers over disputed islands in the East China Sea, in what was a direct challenge to Beijing's newly declared Air Defense Identification Zone.

China declared the ADIZ in an attempt to strengthen its claims in the East China Sea, which are at odds with that of Japan, a major U.S. ally.

Many East Asian governments have strengthened their military ties with Washington, in what is believed to be a response to China's attempt to expand its influence in the region.