China on Monday cautioned against escalating tensions on the Korean Peninsula after a U.S. B-52 bomber flew over South Korea in response to North Korea's nuclear test last week.

The United States and its ally South Korea were discussing on Monday sending more strategic U.S. weapons to the Korean peninsula. North Korea said it set off a hydrogen bomb last Wednesday, its fourth nuclear test since 2006, angering China, the North's main ally, and the United States, which said it doubted the device was a hydrogen bomb.

In a show of force and support for allies in the region, the United States on Sunday sent a nuclear-capable B-52 bomber based in Guam on a flight over South Korea.

China called for all sides to avoid raising tension.

"Safeguarding the peace and stability of northeast Asia accords with all parties' common interests. We hope all relevant parties can maintain restraint, proceed cautiously, and avoid successively escalating tensions," Chinese Foreign Ministry spokesman Hong Lei said at a regular briefing in response to a question about the U.S. B-52 flight.

North Korea has been under UN Security Council sanctions since its first test of an atomic device. After its third test, in 2013, the Security Council took about three weeks to agree a resolution that tightened financial restrictions and cracked down on its attempts to ship and receive banned cargo.

South Korea and Japan used a military hotline for the first time after of North Korea's test, South Korea's defence ministry said, in a sign the North's behaviour is pushing the two old rivals closer together.

South Korea has also resumed anti-North propaganda broadcasts through loudspeakers along the border, a tactic that the North considers insulting. It responded with artillery fire the last time South Korea used the speakers in August.

South Korea also said it would restrict access to the jointly run Kaesong industrial complex just north of the heavily militarized inter-Korean border to the "minimum necessary level" from Tuesday. The complex, where South Korean factories employ North Korean workers, is an important source of revenue for the impoverished North.