Two US Air Force B-1B strategic bombers from Andersen Air Force Base, Guam, conducted training with fighter aircraft from the Japan Air Self Defense Force and a low-level flight with fighter aircraft from the Republic of Korea and the US, in response to the recent nuclear test by North Korea, OSAN airbase, Republic of Korea, September 13. Photo: US Forces Korea

Two huge US bombers flew over South Korea on Tuesday in a show of force against North Korea as a top US envoy said China must help close loopholes in sanctions following Pyongyang's largest-ever nuclear test.

The supersonic B-1B Lancers reached airspace over the US Osan Air Base at Pyeongtaek, 64 kilometers south of Seoul, at around 10:00 am (0100 GMT).

Each aircraft, which had flown from the US Pacific Command's Andersen Airforce base at Guam, was escorted by US and South Korean fighter jets.

"Today's demonstration provides just one example of the full range of military capabilities in the deep resources of this strong alliance to provide and strengthen extended deterrence", said General Vincent Brooks, US-South Korea Combined Forces Command.

"North Korea's nuclear test is a dangerous escalation and poses an unacceptable threat," said the general, adding the US has an "unshakable commitment" to defend allies in the region.

At a regular press conference, Chinese Foreign Ministry spokesperson Hua Chunying suggested Tuesday's show of strength by the US military is not helping the situation.

"China's stand is very clear, that is, under the current situation, we hope all sides should stay rational and restrained, not irritating each other or take any action that will lead to spiral escalations of tension in this region," Hua said.

"If we fall into a cycle of provocations and the build-up of tension, I think it is not in line with anyone's interests," she added.

According to observers, China's stance was hardly a surprise.

"China's interest and ours coincides in the short-term but in not the long-term," said veteran US diplomat and former senate majority leader, George Mitchell.

"China is very worried about the North Korean regime but it does not want a collapse of that regime and a unified Korea under the current the South Korea regime which they would regard as a mortal threat to their interest (with) American security forces right alongside their border," he told CNBC's "Squawk Box".

That explained Beijing's intense opposition to the deployment of the US Terminal High Altitude Air Defense (THAAD) defense system on South Korean soil even though Seoul was an ally which had repeatedly explained that the weapon was for self-defense.

"(There is) a larger sense that China feels that they are in a long term rivalry (with the US)… In my view, there is a troubling chill over US-China relations and I don't see either presidential candidates really offering a sensible way to lower the temperatures in this rivalry. But if China is locked into this rivalry as it sees it, then it is taking measures that we don't like at all," Lyle Goldstein, a professor at the China Maritime Studies Institute at the US Naval War College told CNBC.

South Korea in July announced plans to deploy the THAAD to counter growing nuclear and missile threats from North Korea.

But China, which believes the system's radar could also be used to track its own defense, objected strongly. The plan has also met resistance from residents of the southern county of Seongju, the planned site.

South Korean opposition parties oppose the THAAD deployment, insisting it could escalate a regional arms race and hurt ties with China.

They also say that its usefulness against North Korea's military threats is doubtful.

Speaking in the South Korean capital on Tuesday, Sung Kim, the US envoy on North Korea, said that the US remained open to meaningful dialogue with Pyongyang on ending its pursuit of nuclear weapons.

"Our intention is to secure the strongest possible (UN Security Council) resolution that includes new sanctions as quickly as possible," Kim told a news briefing after meeting his South Korean counterpart.

He said that the US would work with China, North Korea's major diplomatic ally, to close loopholes in existing resolutions, which were tightened with Beijing's backing in March.

"China has been very clear that they understand the need for a new UN security council resolution in response to the latest North Korean nuclear test," Kim said.

"Both sides think that North Korea's nuclear test is not beneficial to peace and stability on the Korean peninsula," China's official People's Daily newspaper said on Tuesday following a high-level China-Russia security meeting in Beijing.

"At present, we must work hard to prevent the situation on the peninsula continuing to escalate, and put the issue of the nuclearization of the peninsula back on the track of dialogue and consultation," it said.