The U.N. Security Council unanimously pushed through a harsh slate of sanctions against North Korea on Wednesday — even winning support from China to slash coal imports from its nuclear-armed neighbor — a day after the Obama administration sought to ease regional concerns over a political crisis gripping South Korea.

Acting in response to Pyongyang’s fifth and largest nuclear test carried out in September, the resolution adopted by the 15-member Security Council also put a ban on North Korean copper, nickel, silver and zinc exports, with the goal of slashing the nation’s export revenue by about 25 percent over the coming year.

The sanctions include a host of other measures cracking down on the country’s access to the international banking system and on North Korea’s export of statues, which have earned the country hard currency mostly through sales to African nations.

Beijing’s embrace of the U.S.-drafted resolution signaled unprecedented cooperation from China, which is believed to be the only country buying North Korean coal and has faced pressure for months from Washington to toughen its posture toward the regime of North Korean leader Kim Jong-un.

Japanese Ambassador Koro Bessho told reporters in New York that the point of the sanctions was to force North Korea back to the negotiating table.

“We are introducing the sanctions, not for the sake of introducing sanctions but in order to change the course of [North Korean] policy. If [North Korea] shows commitment to denuclearization, serious commitment and concrete actions, we are certainly ready to come into dialogue with them and try to solve the situation,” Mr. Bessho said.

But the Obama administration’s success in securing China’s support risks being overshadowed by an entirely separate development on the Korean Peninsula: the damaging influence-peddling scandal facing South Korean President Park Geun-hye, Washington’s closest ally against Pyongyang.

Ms. Park, South Korea’s first female president and the daughter of assassinated former leader Park Chung-hee, is fighting to save her job after prosecutors contended she colluded with a friend, Choi Soon-sil, to enable her to wield improper sway in government affairs and in fundraising by two foundations set up to back the president’s initiatives.

In a dramatic move Wednesday, South Korean opposition parties vowed to press ahead with an attempt to impeach Ms. Park, a day after she announced that she may be willing to resign in exchange for a vaguely defined set of conditions.

Ms. Park on Tuesday asked lawmakers to decide how and when she should quit. But by Wednesday, opposition lawmakers had dismissed the request as a backdoor ploy to slow the political momentum and avoid impeachment.

The leaders of the three opposition parties, which hold enough seats in the single-chamber South Korean parliament to initiate an impeachment motion, said they would not negotiate. The head of one of the parties — the People’s Party — said a motion could be put to a vote as early as Friday.

The fast-paced developments of the scandal have put nerves on edge in Washington, where lawmakers from both sides of the aisle have praised Ms. Park’s hard line against North Korea.

Parallel to the growing international economic sanctions triggered by Pyongyang’s nuclear tests, the Park administration has embraced a significant upgrade in the decades-old Washington-Seoul military alliance: accepting the deployment of a missile defense system despite harsh objections from Pyongyang and Beijing.

Anger over defense system

The Terminal High-Altitude Area Defense system has irked China, which is seen to wield growing influence in South Korea amid expanding trade ties between Beijing and Seoul. Some analysts have raised concern that the scandal surrounding Ms. Park could trigger a political backlash from South Korean opposition parties to the U.S. military presence in the nation.

The Obama administration downplayed such concerns Tuesday. White House spokesman Josh Earnest told reporters that “the security relationship between [South Korea] and the United States is substantial and so important that it supersedes political relationships.”

“Obviously, there is a rather complicated, shall we say, domestic political situation inside of South Korea right now,” Mr. Earnest said. “That is a situation that the South Korean people will grapple with, but the ongoing alliance between our two countries is as strong and durable as ever.”

The White House on Wednesday circulated a statement touting the new U.N. Security Council action against North Korea, saying the world body had effectively strengthened and expanded “sectoral sanctions on exports [that Pyongyang] can use to raise hard currency that can be used to fund its nuclear and ballistic missile programs.”

The council’s new resolution aims to reduce North Korea’s coal exports — the country’s biggest export item — by about 60 percent, with an annual sales cap of $400.9 million.

U.S. Ambassador to the U.N. Samantha Power said the Obama administration is realistic about the limits of the sanctions’ impact. “No resolution in New York will likely, tomorrow, persuade Pyongyang to cease its relentless pursuit of nuclear weapons,” she said. “But this resolution imposes unprecedented costs on the [North Korean] regime for defying this council’s demands.”

Mr. Earnest said Wednesday that the Security Council’s unanimous vote sent a clear signal that it’s not just allies like South Korea and Japan that fear North Korea’s nuclear ambitions. He said the sanctions were imposed as a result of weeks of “hard-nosed diplomacy” that also involved Russia and China.

“Putting in place this hard cap [on coal exports] and closing loopholes they had previously exploited to get around previous sanctions is a substantial development,” Mr. Earnest said.

It remains to be seen whether China, considered the North’s only real ally, will implement the sanctions. Over the first 10 months of 2016, the Chinese have imported 18.6 million tons of coal from North Korea, up almost 13 percent from last year, according to Reuters. Such imports will face a significant cut under the sanctions.

While Chinese officials have opposed North Korea’s recent nuclear tests, Ambassador Liu Jieyi seemed to suggest Wednesday that Beijing remains wary about following Washington’s lead toward containing Pyongyang.

Mr. Liu accused the U.S. and South Korea of intensifying confrontation with North Korea through increased military exercises and posturing. He specifically described the THAAD deployment as “neither conducive to the realization of the goal of denuclearization of the Korean Peninsula nor helpful to the maintenance of peace and stability on the peninsula.”

North Korea has been under U.N. sanctions since 2006 as a result of its testing of nuclear devices and ballistic missiles. U.N. Secretary-General Ban Ki-moon told the Security Council on Wednesday that “sanctions are only as effective as their implementation.”

“It is incumbent on all member states of the United Nations to make every effort to ensure that these sanctions are fully implemented,” Mr. Ban said.

• This article is based in part on wire service reports.

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