US President Barack Obama signed an order Wednesday implementing UN-backed sanctions on North Korea after a nuclear test and missile launch this year, as Pyongyang promised reprisals.

The White House said Obama had signed an executive order targeting the volatile hermit state's energy, financial and shipping assets.

The measures were agreed to at the United Nations in response to the January 6 nuclear test and February 7 ballistic missile launch.

"The order is not targeted at the people of North Korea, but rather is aimed at the government," said the document signed by Obama.

Among the entities targeted are the "Propaganda and Agitation Department" of the Workers' Party of Korea and mining firms that provide the regime with much-needed revenues.

The US Treasury Department estimates that coal revenues alone generate over $1 billion a year for the government of Kim Jong-Un.

In response to the UN sanctions and a US-South Korean drill, Kim has already ordered an upcoming nuclear warhead test and multiple ballistic missile launches.

US officials say the threats are concerning, but fit a pattern of sabre rattling by the regime.

Analysts and diplomats have said that loopholes in the UN sanctions leave room for China, Pyongyang's key economic supporter, to continue business as usual.

In 2014, China accounted for more than 90 percent of North Korea's $7.61 billion in total trade, according to the latest available figures from South Korea's state-run Korea Trade-Investment Promotion Agency.

In response to Obama's executive order, Beijing said Thursday that it "opposes any country's unilateral sanctions."

"We have stressed that the unilateral actions taken by any country must not undermine the lawful rights and interests of China," foreign ministry spokesman Lu Kang told a regular briefing.

On Wednesday, North Korea jailed a 21-year-old American student.

Otto Warmbier was sentenced to 15 years' hard labor for stealing a propaganda banner from a hotel.

White House spokesman Josh Earnest accused Pyongyang of using US citizens as "pawns to pursue a political agenda" and called for his release.

"We strongly encourage the North Korean government to pardon him and grant him special amnesty and immediate release," Earnest said.

"The allegations for which this individual was arrested and imprisoned would not give rise to arrest or imprisonment in the United States or in just about any other country in the world."

In announcing the sentence, state news outlet KCNA said Warmbier had committed his offense "pursuant to the US government's hostile policy" toward North Korea.