Stricter international sanctions will only lead North Korea to speed up its nuclear programme, Pyongyang said, as US and Chinese leaders agreed to "maximise the pressure" on the regime of Kim Jong-un.

"The increased moves of the US and its vassal forces to impose sanctions and pressure on the DPRK will only increase our pace towards the ultimate completion of the state nuclear force," a statement on North Korean state media said on Monday, using the acronym for the country's official name, Democratic People's Republic of Korea.

The UN Security Council last week imposed new sanctions on North Korea over its nuclear and ballistic missile programmes, seeking to curb Pyongyang's sources of income.

The new measures slapped Pyongyang with an export ban on textiles, froze work permits to North Korean guest workers, and placed a cap on oil supplies.

The sanctions came on top of measures issued last month that were expected to cut roughly $1bn from the regime's $3bn in annual export revenue.

State news agency KCNA, quoting a foreign ministry statement, said the economic restrictions were an "act of hostility to physically exterminate the people of" North Korea.

"[The US is] disseminating the fraudulent claim that the sanctions and pressure are geared to the so-called peaceful solution," it said.

US Secretary of Defense James Mattis played down the threat of North Korea's recent missile launches.

"Number one, those missiles are not directly threatening any of us," Mattis said on Monday.

The North Koreans "are intentionally doing provocations that seem to press against the envelope for just how far can they push without going over some kind of a line, in their minds, that would make them vulnerable", he said. "So they aim for the middle of the Pacific Ocean."

"The bottom line is that the missiles, were they to be a threat" either to the US or Japan, "that would elicit a different response from us".

US President Donald Trump discussed North Korea's "continued defiance of the international community" with President Xi Jinping, the White House said on Monday.

The two leaders are "committed to maximising pressure on North Korea through vigorous enforcement of United Nations Security Council resolutions", a statement said.

China has traditionally been North Korea's closest international ally, although North Korea's repeated missile tests have hardened Beijing's line on the Kim regime.

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The international community is scrambling to contain an increasingly belligerent Pyongyang, which has conducted its sixth and largest nuclear test and fired long-range missiles over Japan that it says could reach the US mainland.

North Korea says it needs nuclear weapons to protect itself from US forces and is determined to build a system capable of delivering a nuclear warhead capable of hitting the mainland US.

North Korea is expected to be a main talking point at the UN General Assembly in New York this week.