Washington (CNN) Two US B-1 bombers flew over disputed waters in the East and South China Seas on Thursday, conducting a joint military operation with Japanese fighter jets amid escalating tensions with North Korea and souring relations with China.

The bombers were joined by two Japanese F-15 fighters and carried out a cooperative mission over the East China Sea -- an area both Japan and China claim as their own.

While joint flights between the two allied nations have become increasingly routine, this mission marked the first time US B-1 bombers from the Pacific Command have carried out an operation of this kind with Japanese fighters at night, according to a statement from US Pacific Air Forces.

"Flying and training at night with our allies in a safe, effective manner is an important capability shared between the US and Japan," said Maj. Ryan Simpson, Pacific Air Forces chief of bomber operations.

"This is a clear demonstration of our ability to conduct seamless operations with all of our allies," he added.

The Japanese Air Self Defense Force claims that the mission was not intended to send a message to any specific country despite previous face-offs in the East China Sea with Chinese ships and warplanes.

Earlier this year, US Defense Secretary James Mattis reaffirmed US commitment to defending Japan and its disputed islands.

Following the joint operation, the US B-1 bombers then flew over the South China Sea "to exercise the rights of freedom of navigation" before returning to Andersen Air Force Base in Guam.

The US has routinely challenged China's claims to sovereignty in the South China Sea and the issue has put a strain on relations between the two powerful nations for years.

"We have noted relevant reports but I have no information on this specific case, said Geng Shuang, a Chinese foreign ministry spokesman on Friday.

"China has always respected and supported other countries' freedom of navigation of overflight in accordance with international law," he added. "But we are firmly opposed to saber-rattling that harms China's sovereignty and security by certain countries on the pretext of freedom of navigation and overflight."

On Sunday, a US Navy destroyer sailed within 12 miles of a disputed island in the South China Sea that is claimed by China, a US military official told CNN.

China called the action "a serious political and military provocation." The US "stirs up trouble" and runs "in the opposite direction from countries in the region who aspire for stability, cooperation and development," the ministry statement said.

End of the honeymoon and North Korea

Thursday's demonstration of military might by the US and one of its key regional allies comes on the heels of a tense week in US-China relations, which analysts said signaled the end of the honeymoon period between the two nations that began with a summit at President Donald Trump's Mar-a-Lago resort in April.

Xi said that ties with the US have been strained by "some negative factors" in a telephone conversation with Trump on Sunday following a flurry of controversial moves from the US.

"Xi Jinping stressed that since his meeting with President Trump, important results have been achieved in China-US relations," reported Chinese state media outlet CCTV.

"Meanwhile, bilateral relations have also been affected by some negative factors, for which the China side has expressed its position to the US side."

Last week, a $1.4 billion arms sale to Taiwan , which Beijing regards as a renegade province, was approved after months of delay, the same day as China's Dandong bank was sanctioned by the US for alleged ties to North Korea.

The US also labeled China as one of the world's worst human traffickers and challenged Beijing in the South China Sea, by sailing close to a disputed island chain that China claims.

Many took these moves as evidence of the US pursuing a harder line on the Asian superpower, likely caused by mounting frustration over a perceived lack of action by Beijing to contain North Korea.

According to the White House statement, Trump raised the growing threat of North Korea's nuclear and ballistic arsenals with the Chinese president in their phone call on Sunday.

"Both leaders reaffirmed their commitment to a denuclearized Korean Peninsula," the White House said in a readout of the phone call.

Trump also spoke to Japanese Prime Minister Shinzo Abe by phone on Sunday, with the two pledging deeper cooperation on North Korea in the wake of growing frustrations over the rogue state.

But North Korea's successful launch of an intercontinental ballistic missile on Tuesday has raised the stakes ahead of Trump and Xi's scheduled meeting at the G20 Summit on Saturday as relations between China and the United States continue to cool in the months following the February's cordial meeting between the two leaders.

Not giving up hope

Secretary of State Rex Tillerson said Friday that the United States has not given up hope on China helping to solve the situation in North Korea, calling US efforts to persuade China to help a "peaceful pressure campaign."

Tillerson's statement contrasted with some of Trump's tweets, where the President has indicated he had largely given up on China's willingness to help in North Korea.

"No, we have not given up hope. I call it the peaceful pressure campaign," Tillerson said. "This is a campaign to lead us to peaceful resolution because if this fails we don't have very many good options left."

Tillerson said China's contributions have been "a bit uneven."

"China has taken significant action and then I think, for a lot of different reasons, they paused and didn't take additional action. They then have taken some steps and then they paused. ... We have remained very closely engaged with China both through our dialogues that has occurred face-to-face but also on the telephone," he said.

Tillerson added, "There is a clear understanding between the two of us on our intent and I think the sanctions action that were taken here just in the last week or ten days certainly going their attention in terms of their understanding of our resolve to bring more pressure to bear by directly going after entities doing business with North Korea, regardless of where they may be located."