North Korea's government has heavily criticised new economic sanctions by the United States, calling them an "act of war" on Sunday.

"Like we have said repeatedly, we would consider any restrictions on us as an act of war, and we will not stop the US if it really has the nerves to confront us in a 'rough' manner," North Korea's foreign ministry said according to KCNA, the country's state-run news agency.

"The two Koreas have cooperated together and the Olympics was held successfully," the statement continued.

"But the US brought the threat of war to the Korean Peninsula with large-scale new sanctions on the DPRK (Democratic People's Republic of Korea) ahead of the Olympics closing ceremony."

The US government announced the new sanctions, which aim to prevent North Korea from further developing its nuclear programme, on Friday.

The sanctions, introduced on Friday by US President Donald Trump, prohibit US citizens from dealing with more than 50 vessels and companies, and one person, located in countries including North Korea, China, Taiwan and Hong Kong.

Assets held by the firms within the US will also be blocked.

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Trump called the measures the "heaviest sanctions ever imposed on a country before" in an address in Washington on Friday.

He cautioned the US will "have to go to phase two" if the sanctions don't have Washington's desired effect".

Steven Mnuchin, the US treasury secretary, said the measure "will significantly hinder the Kim regime's capacity to conduct evasive maritime activities".

"[Those activities] facilitate illicit coal and fuel transports, and erode [North Korea's] abilities to ship goods through international waters," Mnuchin added.

"The president has made it clear to companies worldwide that if they choose to help fund North Korea's nuclear ambitions, they will not do business with the United States."

China, North Korea's closest ally, said in a response to the US sanctions that "unilateral actions" could undermine cooperation between Beijing and Washington.

The Chinese foreign ministry said it had lodged "stern representations" with the US over the measures.

"The Chinese side firmly opposes the US imposing unilateral sanctions and long-arm jurisdiction on Chinese entities or individuals in accordance with its domestic laws," Geng Shuang, a foreign ministry spokesperson, said.

The new US sanctions come two months after the UN Security Council said it was imposing its toughest sanctions yet on North Korea.

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The Security Council unanimously voted to ban nearly 90 percent of refined petroleum exports to North Korea, and order North Koreans who work abroad to return to the country within 24 months, on December 22.

North Korea announced in November it had successfully conducted a new intercontinental ballistic missile (ICBM) capable of reaching the US mainland.

North Korean leader Kim Jong-un's government conducted several missile tests last year, drawing condemnation from the international community.

Tensions on the Korean Peninsula appear to have eased in the last few weeks, however, with South Korea expressing it was cautiously optimistic of making progress with inter-Korean relations in the wake of a visit by North Korean officials during the Winter Olympics in Pyeongchang.