South Korea warned North Korea on Wednesday that the United States and its allies were working on sanctions to inflict "bone-numbing pain" after its latest nuclear test, and called on China to do its part to rein in its isolated neighbour.
The US House of Representatives voted nearly unanimously late on Tuesday to pass legislation to broaden sanctions on the North's nuclear programme.
Meanwhile, North Korean leader Kim Jong Un called for an expansion of the size and power of his isolated country's nuclear arsenal, state media reported on Wednesday.
Last week's nuclear test was North Korea's fourth, although the United States and experts doubt the North's claim that it was of a more powerful hydrogen bomb, as the blast was roughly the same size as that from an atomic bomb test in 2013.
South Korean President Park Geun-hye said further provocations by the North including "cyber-terrorism" were possible, and said new sanctions should be tougher than those in the past, but did not give specifics.
"We are cooperating closely with the United States and allies to come up with effective sanctions that will make North Korea feel bone-numbing pain, not only at the Security Council but also bilaterally and multilaterally," she said in a speech.
Park said Seoul and Beijing were discussing a draft UN Security Council resolution on North Korea, noting that Beijing has stated repeatedly that it would not tolerate the North's nuclear programme.
China is the North's main ally and trade partner.
"I am certain that China is very well aware if such a strong will isn't followed by necessary steps, we will not be able to stop the North's fifth and sixth nuclear tests and we cannot guarantee true peace and stability of the Korean peninsula," she said.
South Korea and China have grown increasingly close in recent years.
"I believe the Chinese government will not allow the situation on the Korean peninsula to deteriorate further," she said. "The best partners are those that will hold your hand in difficult times."
Last week, US Secretary of State John Kerry said he had told his Chinese counterpart that China's approach to North Korea had not succeeded.
While China last week urged North Korea to stick to its denuclearisation pledges and avoid action that would make the situation worse, it also said it did not hold the key to resolving the North Korean nuclear issue.
Later on Wednesday, South Korea's top nuclear envoy will meet his US and Japanese counterparts in Seoul and travel to Beijing to meet his opposite number on Thursday. South Korea's foreign minister was to speak by phone with his Russian counterpart later on Wednesday to discuss the situation.
US sanctions vote
The US House sanctions measure passed by 418-2 on Tuesday and Senate leaders expect to consider a similar bill shortly.
The House bill had been introduced in early 2015, but was not brought up for a vote until after Pyongyang announced last Wednesday it had tested a hydrogen bomb.
"(The bill) uses targeted financial pressure to isolate (North Korean leader) Kim Jong Un and his top officials from the assets they maintain in foreign banks, and from the hard currency that sustains their rule," said Republican Representative Ed Royce, chairman of the House Foreign Affairs Committee and an author of the measure.
To become law, the legislation must be passed by the US Senate and signed by President Barack Obama.
In Pyongyang, Kim called for a bolstering of the country's nuclear arsenal and the "detonation of more powerful H-bomb in the future", according to state media.
