Twice in just three days, the US sent heavy bombers through the disputed South China Sea, sending a message to US rivals in the region.

The US did the same last month, sparking criticism in Beijing, which claims the vast majority of the South China Sea.

Secretary of Defense James Mattis argues that these flights would not mean a thing to anyone if China had not militarized the waterway.

The US Air Force sent B-52H Stratofortress heavy long-range bombers through the South China Sea twice this week, sending a message, intentional or not, to challengers in the region.

A single B-52 bomber assigned to the 96th Expeditionary Bomb Squadron conducted training in the South China Sea and Indian Ocean on Sunday, Pacific Air Forces Public Affairs told Business Insider on Wednesday. Two days later, another B-52 bomber conducted a training mission in the South China Sea.

"U.S. Indo-Pacific Command's Continuous Bomber Presence (CBP) operations have been ongoing since March 2004," PACAF told BI, adding that these recent missions are "consistent with international law and United States's long-standing and well-known freedom of navigation policies."

"The United States military will continue to fly sail and operate wherever international law allows at a times and places of our choosing," Pentagon spokesman Lt. Col. Dave Eastburn told Business Insider on Tuesday.

While Beijing has yet to criticize the bomber flights, Secretary of Defense James Mattis stressed Wednesday that if China has a problem with these flights, it will be because China made it a problem through its activities in contested waters.

"If it was 20 years ago and had they not militarized those features there it would have been just another bomber on its way to Diego Garcia or wherever," the secretary explained. "There's nothing out of the ordinary about it."

Last month, the US sent B-52s through the East and South China Sea four times, twice in each waterway. The US also sent B-52s through the South China Sea in April and June, prompting the Chinese Ministry of Foreign Affairs to accuse the US of "running amok" in the region.

The latest flights come at a time of rising tension between Washington and Beijing.

Not only are the US and China locked in an escalating and intensifying trade war involving tariffs on hundreds of billions of dollars in goods, but tensions are also causing military-to-military relations to deteriorate.

Last week, the US sanctioned a procurement division of the Chinese military for purchasing Russian weapons systems in violation of sanctions, namely the advanced Su-35 fighter jet and the S-400 surface-to-air missile system. China then suddenly canceled a meeting between Vice Admiral Shen Jinlong and his US counterpart, Chief of Naval Operations Adm. John Richardson. Beijing also rejected a request by the US Navy to permit a port call by the amphibious assault ship USS Wasp in Hong Kong.

Analysts and experts suspect that it will be a long time before military ties recover.