North Korea has reportedly fired another missile, bringing the total number of launches in the past three days to six, in direct defiance of US warnings that it would "pay a price" for the nuclear and missile tests.

The move follows the firing of two short-range missiles off its east coast on Tuesday, hours after the UN security council condemned the apparently successful test of a nuclear weapon as powerful as the one that destroyed Hiroshima.

The UN security council has begun behind-the-scenes negotiations designed to strengthen sanctions against Pyongyang, following an emergency meeting. Diplomats scrambled to forge a united front against a test seen as a provocative step towards North Korea gaining a full nuclear arsenal.

Council members said they would follow up with a new legally binding resolution. Earlier, the US ambassador to the UN, Susan Rice, said the international community would not "throw up our hands and let them pursue this path" and that North Korea would "pay a price for their action".

South Korea's mass-circulation Chosun Ilbo newspaper reported that US spy satellites have detected steam coming from a reprocessing facility at North Korea's main Yongbyon nuclear plant, indicating the North has restarted the reprocessing of spent nuclear fuel rods to harvest weapons-grade plutonium.

Tensions on the peninsula rose earlier as Pyongyang accused the US of "hostile intent" and the South Korean news agency Yonhap said the North had fired two missiles, citing an unnamed government source.

Seoul announced it would immediately join a US-led initiative to intercept ships suspected of carrying nuclear, chemical and biological weapons, component parts or missiles to deliver them. Pyongyang has warned it would consider membership a declaration of war.

Barack Obama spoke at the White House last night, denouncing North Korea's action as "a blatant violation of international law". He said North Korea "will not find security and respect through threats and illegal weapons".

Japan, which considers itself high on Pyongyang's potential hit list, said it would seek a fresh UN resolution condemning the test.

Gordon Brown described the test as "erroneous, misguided and a danger to the world".

The UN security council talks in New York produced no fresh initiative beyond a statement condemning the test. But there were signs that several members were prepared to press for tightened sanctions when the hard work begins

seeking a resolution that would be acceptable to all parties.

Susan Rice, the US ambassador to the UN, made clear she would urging fresh punitive moves, saying the US would seek "a strong resolution with strong measures".

All eyes will now turn to China, North Korea's biggest ally, which has in the past been wary to tighten the screw on its neighbour. Despite its cautious stance, Beijing said it "resolutely opposed" the test and urged Pyongyang to avoid actions that would sharpen tensions and to return to six-party disarmament talks.

Early on Monday, South Korea said it had detected a tremor consistent with those caused by an underground nuclear explosion. Yonhap reported that North Korea also test-fired three short-range missiles from a base on the east coast immediately after the nuclear test. The underground atomic explosion, at 9.54am local time (0154 BST), created an earthquake measuring magnitude 4.5 in Kilju county, in the country's north-east, reports said.

Russian defence experts estimated the explosion's yield at between 10 and 20 kilotons, many times more than the 1 kiloton measured in its first nuclear test in 2006 and about as powerful as the bombs the US used against Hiroshima and Nagasaki at the end of the second world war. One kiloton is equal to the force produced by 1,000 tonnes of TNT.

The force of the blast made the ground tremble in the Chinese border city of Yanji, 130 miles away.

North Korea had warned of a second nuclear test after the UN condemned its ballistic missile test on 5 April and agreed to tighten sanctions put in place in 2006. Pyongyang said it had put a communications satellite in orbit, but experts said the technology and methods were identical to those used to launch the long-range Taepodong-2 missile.

After the UN refused to apologise for condemning the launch North Korea expelled international inspectors, threatened to restart its Yongbyon nuclear reactor – which it had agreed to start dismantling in 2007 – and walked away from the six-party nuclear talks.

The KNCA news agency, the regime's official mouthpiece, said yesterday: "We have successfully conducted another nuclear test on 25 May as part of the republic's measures to strengthen its nuclear deterrent."

The agency said the test had been "safely conducted on a new higher level in terms of its explosive power and technology of its control".

A commentary carried by KCNA accused the US government of following the Bush administration's "reckless policy" towards the North, warning: "The US would be well advised to halt at once its dangerous military moves against the DPRK [North Korea] if it wants to escape the lot of a tiger moth, bearing deep in mind that any attempt to make a pre-emptive attack on the DPRK is little short of inviting a disaster itself."

The test will add to fears that North Korea is moving closer to possessing the ability to put a nuclear warhead on long-range missiles that are capable, in theory, of reaching Hawaii and Alaska.

Analysts believe the North Korean leader, Kim Jong-il, hopes to use the test to shore up support from the military amid speculation that he is about to name one of his three sons as his successor.

Kim, 67, appears to be re-establishing his grip on power since reportedly suffering a stroke last August. While diplomats around the world debated how to respond to yesterday's test, the North Korean leader was enjoying a military performance including songs such as Our General Is the Best and Song of Coastal Artillery Women, according to state media.