SEOUL (Reuters) – 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 neighbor.

With tension high on the border after the North’s fourth nuclear test on Wednesday last week, South Korean forces fired shots toward what Yonhap News Agency said was a suspected North Korean drone.

It returned to the North after the shots, South Korean military officials told Reuters.

The North’s nuclear test has angered both China and the United States and again raised questions about what can be done to stop its development of nuclear weapons.

The World Economic Forum withdrew its invitation for North Korea’s foreign minister to attend its annual Davos meeting, which was to have been the country’s first participation in the event in 18 years, because of the nuclear test.

North Korea said it had tested a powerful hydrogen bomb but the United States and various experts doubt that, as the blast was roughly the same size as that from its previous test, of an atomic bomb, in 2013.

The U.S. House of Representatives voted nearly unanimously on Tuesday to pass legislation to broaden sanctions on the North.

But apparently unperturbed by the prospect of further international isolation, North Korean leader Kim Jong Un called for an expansion of the size and power of his country’s nuclear arsenal, urging the “detonation of more powerful H-bombs”, the North’s state media reported.

South Korean President Park Geun-hye said more “provocations” by the North including “cyber-terrorism” were possible and new sanctions should be tougher than previous ones. She 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 South Korea and China were discussing a U.N. Security Council resolution on North Korea, noting that China has stated repeatedly that it would not tolerate the North’s nuclear program.

China is the North’s main ally and trade partner but it has made clear it opposes its bombs, while China’s ties with South Korea have grown increasingly close in recent years.

“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,” Park said.

“I believe the Chinese government will not allow the situation on the Korean peninsula to deteriorate further.”

Sung Kim, the special U.S. representative for North Korea policy, met with his South Korean and Japanese counterparts in Seoul on Wednesday and said the three agreed that a “meaningful” new sanctions resolution is needed from the Security Council.

“I hope the Chinese authorities agree with us that we simply cannot take a business as usual approach to this latest provocation. We will be working very closely with them to come up with a meaningful resolution,” he said.

‘FINANCIAL PRESSURE’

China rejects complaints it is not doing enough on North Korea. In Beijing, Foreign Ministry spokesman Hong Lei said China’s efforts toward a denuclearized Korean peninsula would continue.

“This is in everyone’s interests and is everyone’s responsibility, including China and South Korea,” he said.

The U.S. House sanctions measure passed by 418-2 and Senate leaders expect to consider a similar bill shortly.

The House bill had been introduced in 2015 but was not brought up for a vote until after North Korea’s latest test.

“(The bill) uses targeted financial pressure to isolate Kim Jon 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, it must be passed by the U.S. Senate and signed by President Barack Obama.

The 28,500 U.S. troops in South Korea have been put on high alert as a noisy propaganda battle is played out across the heavily fortified border with the North.

South Korea, still technically at war with the North since their 1950-53 conflict ended in a truce, not a treaty, has for days been blaring propaganda through loudspeakers across the border.

South Korea’s military said it had found anti-South leaflets in the Seoul area, which it suspects were dropped by North Korean hot air balloons.

South Korean financial regulators met computer security officials at 16 banks and financial institutions and urged vigilance in the face of possible cyber attacks by North Korea, although none has been detected.

(Additional reporting by Jack Kim, James Pearson, Jee Heun Kahng, Hooyeon Kim, Dahee Kim and Se Young Lee in SEOUL, Tom Miles in GENEVA, Patricia Zengerle in WASHINGTON and Ben Blanchard in BEIJING; Editing by Robert Birsel and Nick Macfie)