North Korea is already under multiple UN sanctions but has still made rapid progress in its nuclear and missile programmes

North Korea vowed Wednesday to accelerate its weapons programmes in response to "evil" sanctions imposed by the UN Security Council following its latest and most powerful nuclear test.

The respected 38 North website in the US raised its estimate for the yield from the explosion, which Pyongyang says was a hydrogen bomb small enough to fit onto a missile, to around 250 kilotons -- more than 16 times the size of the device that devastated Hiroshima in 1945.

The detonation, Pyongyang's sixth nuclear blast, prompted global condemnation and came after it carried out two intercontinental ballistic missile launches in July that appeared to bring much of the US into range.

North Korea has shrugged off numerous sets of sanctions aimed at crimping its nuclear and missile programmes, and this month unveiled what it said was a working hydrogen bomb

The UN Security Council unanimously imposed an eighth set of sanctions on the North Monday, banning it from trading in textiles and restricting its oil imports, which US President Donald Trump said was a prelude to stronger measures.

The resolution, passed after Washington toned down its original proposals to secure backing from China and Russia, came just one month after the council banned exports of coal, lead and seafood in response to the ICBM launch.

The North's foreign ministry condemned the new measures "in the strongest terms", calling them a "full-scale economic blockade" driven by the US and aimed at "suffocating" its state and people.

North Korea recently tested two intercontinental ballistic missiles (ICBM) that appear to bring much of the US within their maximum 10,000 kilometre range

It was "another illegal and evil 'resolution on sanctions' piloted by the US", it said in a statement carried by the official KCNA news agency.

"The DPRK will redouble the efforts to increase its strength to safeguard the country's sovereignty and right to existence," the ministry said, using the abbreviation for the North's official name.

- Radioactive gas -

But the South's unification ministry described the statement as "the most low-key form of response from North Korea to UN Security Council resolutions".

Seoul conducted its first live-fire exercise of its new long-range Taurus missile in response to the nuclear test, its Air Force said.

South Korean warplanes recently conducted live-fire exercises in response to North Korea's sixth nuclear test

The German air-to-surface weapon was capable of precision strikes on key North Korean facilities even if launched from the central part of the South, it added.

The US and its allies argue that tougher sanctions will pile pressure on North Korea to negotiate an end to its weapons programmes but experts are sceptical.

US President Donald Trump said the latest measures were a "very small step - not a big deal" that must lead to tougher measures.

Military balance in northeast Asia

"Those sanctions are nothing compared to ultimately what will happen," Trump said, but added that it was "nice to get a 15 to nothing vote".

The North says it needs nuclear weapons to protect itself from "hostile" US forces and analysts believe Pyongyang's weapons programme has made rapid progress under leader Kim Jong-Un, with previous sanctions having done little to deter it.

Government estimates of the yield from its sixth nuclear test vary from South Korea's 50 kilotons to Japan's 160, but 38 North, which is linked to Johns Hopkins University in the US, raised its estimate to "roughly 250 kilotons", in line with upward revisions for the magnitude of the resulting tremor.

South Korea's Nuclear Safety and Security Commission said Wednesday it had collected a small amount of xenon-133 -- a radioactive isotope of the inert gas that does not occur naturally -- that was "linked to the latest nuclear test".

But the commission said in a statement it was "unable to confirm what type of nuclear test was conducted".

Washington had initially sought a full oil embargo and a freeze on the foreign assets of leader Kim Jong-Un in response to the blast, but dropped them following strong opposition from China and Russia.

The new resolution instead bans trade in textiles, cuts off natural gas shipments to North Korea, places a ceiling of 2 million barrels a year on deliveries of refined oil products and caps crude oil shipments at current levels.

Retail petrol prices in the North jumped earlier this year, with some analysts suggesting the authorities were stockpiling in the expectation of a ban.

According to the US mission to the United Nations, the North imports around 8.5 million barrels a year of oil and oil products, 4 million as crude and 4.5 million in refined form -- which includes substances such as petrol and diesel.

It added that the North's textile exports averaged $760 million a year.

The UN resolution also barred countries from issuing new authorisations to North Korean workers sent abroad. There are almost 100,000 of them, according to the US mission, earning more than $500 million a year for the regime.

Under the measure, joint ventures with North Korean entities are prohibited, while governments are authorised to inspect ships suspected of carrying banned cargo from the country, but must first seek the consent of the vessels' flag state.