Two US Navy destroyers conducted a freedom-of-navigation operation in the South China Sea on Monday, challenging China's claims to the Spratly Islands.

Beijing accused the US of entering Chinese waters without permission and engaging in provocations that threaten China's sovereignty.

It was the second such US operation in the South China Sea this year.

Freedom-of-navigation operations occur frequently, but they have not stopped China from bolstering its position militarily in the South China Sea.

The US Navy sent two guided-missile destroyers to challenge China in the South China Sea, and Beijing is outraged.

Two Arleigh Burke-class destroyers — the USS Spruance and the USS Preble — conducted a freedom-of-navigation operation on Monday, sailing within 12 nautical miles of Chinese outposts in the contested Spratly Islands.

The purpose was "to challenge excessive maritime claims and preserve access to the waterways," as well as to show that the US "will fly, sail, and operate wherever international law allows," Cmdr. Clay Doss, a US Navy 7th Fleet spokesman, told CNN.

Beijing sharply criticized the operation. A spokeswoman for the Chinese foreign ministry, Hua Chunying, accused the US of entering Chinese waters without permission and engaging in provocations that threaten China's sovereignty, the South China Morning Post reported.

China's claims to the South China Sea were largely discredited by an international arbitration tribunal three years ago. Beijing rejected the ruling and the authority of the tribunal.

The Chinese military has since bolstered its presence in the region through deploying surface-to-air missiles, anti-ship missiles, jamming technology, and other defense systems to Chinese-occupied territories.

Monday's sail-by is the second US freedom-of-navigation operation in the South China Sea this year. In January, the destroyer USS McCampbell challenged China's claims to the Paracel Islands.

Read more: The US Navy sent the destroyer USS McCampbell and replenishment oiler USNS Walter S. Diehl through the Taiwan Strait

China then accused the US of trespassing, and Chinese media announced it had deployed a "far-reaching anti-ship ballistic missile," the DF-26, that was "capable of targeting medium and large ships."

Read more: China moves 'ship killer' missiles into firing range as US warships infuriate Beijing in the South China Sea

The US has also sailed warships through the Taiwan Strait, drawing Beijing's ire.

During a South China Sea freedom-of-navigation operation in September, a Chinese destroyer challenged a US vessel to a showdown, forcing the US Navy ship off course and risking a deadly collision.

The Chinese foreign ministry on Monday said the People's Liberation Army Navy sent vessels to track the US warships sailing near the Spratlys and warned them to leave the area.

The US Navy, however, has stressed that it's not going anywhere. "We've got big interests there, so we're going to remain there," Adm. John Richardson, the chief of naval operations, said recently.