North Korea has issued a fresh warning to neighbour South Korea less than 24 hours of threatening to target the US.

As the war of words escalates, Pyongyang has vowed to take "physical countermeasures" against Seoul if it takes part in the recently imposed UN sanctions which further tighten the screws on the defiant Communist nation.

"'Sanctions' mean a war and a declaration of war against us. If the South takes direct part in the UN sanctions, [North Korea] will take strong physical countermeasures against it," said Pyongyang.

A statement from the government, carried by the official KCNA news agency, said: "Now that the South Korean conservative groups are desperately kicking up a racket against the DPRK over its nuclear and rocket issue with the United States, there will be no more discussions on the denuclearisation between the North and South in the future.

"As long as the South Korean group of traitors persistently pursues a hostile policy toward the DPRK, we will never negotiate with anyone."

North Korea had earlier declared that its third nuclear test was aimed at Washington, prompting sharp reactions from the US.

North Korea's contention that imposing sanctions is tantamount to a declaration of war is bound to further heighten tensions in the region, according to analysts.

The UN Security Council's unanimous resolution, backing sanctions against North Korea, reiterated the council's previous demands that North Korea abandon its nuclear weapons programme and refrain from further rocket launches.

The latest sanctions, aimed at "targeting entities and individuals" involved in the nuclear and weapons programmes include a blanket travel ban, together with curbs on North Korea's vital space agency, a bank, and on a handful of firms. Four key individuals have been added to the existing list.

According to the resolution, the UN will initiate "significant action" if Pyongyang presses ahead with its third nuclear test.

The resolution was the fifth to declare sanctions on North Korea in relation to its nuclear programme since 1993.

Apart from the UN-backed sanctions list, Washington has issued its own restrictions which include the blacklisting of two individuals from North Korean banks based in Beijing and a Hong Kong-based trading firm. The trading company was already on the UN blacklist.

North Korea survived the previous economic sanctions by boosting its trade ties with its only diplomatic partner, China. However Beijing has supported the new sanctions against North Korea, which could prove crippling.