Two huge US bombers flew over South Korea Tuesday in a show of force against the North 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 kilometres (40 miles) 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.

The US "will take necessary steps to do so, including operations like this one today, and the deployment of the Terminal High Altitude Air Defense (THAAD) battery to the Korean Peninsula," he said.

South Korea in July announced plans to deploy THAAD, a sophisticated US missile defence system, to counter growing nuclear and missile threats from the North.

But China, which believes the system's radar could also be used to track its own defences, 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 its usefulness against the North's military threats is doubtful.

- 'Clear warning' -

Tuesday's overflight was "aimed at sending a clear warning to North Korea about its nuclear test, and containing further provocations," South Korea's defence ministry spokesman Moon Sang-Gyun told journalists.

Washington took similar actions after previous atomic tests.

In January, the US flew a B-52 Stratofortress strategic bomber over Osan Air Base after North Korea's fourth nuclear blast.

South Korea is home to around 28,500 US troops and numerous bases.

The flight came after the North on Friday carried out what it described as a "nuclear warhead test" and vowed to take further measures to increase its nuclear strike force "in quality and in quantity".

Washington's special representative for North Korea policy, Sung Kim, said in Seoul Tuesday that China must help the international community close loopholes in sanctions against Pyongyang.

The UN Security council is scrambling to come up with new penalties, but five sets of UN sanctions since North Korea first tested a nuclear device in 2006 have failed to deter it.

Beijing has said it opposes the testing, but analysts believe it pulls its punches because it is desperate to avoid anything that would imperil the status quo and alter the balance of power on the Korean peninsula in favour of the US.

China -- Pyongyang's sole ally and largest benefactor -- had a key role to play in showing North Korea the "serious consequences for its unlawful and dangerous actions," Kim said.

"We look forward to working with Beijing, to... try to close any loopholes" in the latest sanctions, Kim told journalists.