North Korea said its nuclear test Tuesday was only a "first" step and warned of stronger action if it was faced with tougher sanctions as a result.

"The latest nuclear test was only the first action, with which we exercised as much self-restraint as possible," the foreign ministry said in a statement carried by the country's official news agency.

A North Korean state media outlet has accused the United States of "jumping to conclusions" that the North would soon stage a nuclear test, adding to the confusion over its immediate intentions.

The U.S. and its ally South Korea are "fussing over speculation" without knowing exactly what action the North plans to take, Tongil Sinbo, a Japan-based pro-North weekly magazine funded by Pyongyang, said in an editorial.

U.N. leader Ban Ki-moon warned Thursday that a nuclear test by North Korea could blow up hopes of an eventual reconciliation by "tying the hands" of the South's incoming president.

Ban, a former South Korean foreign minister, said incoming South Korean president Park Geun-Hye is "very much committed" to improving relations with North Korea.

North Korea has vowed to carry out a third nuclear test, but scientists and concerned foreign governments may have a tough time verifying the actions of the reclusive state.

One critical question is whether North Korea uses uranium or plutonium. North Korea's 2006 and 2009 tests involved plutonium, so a uranium detonation would prove that Kim Jong-Un's regime has opened an additional way to make bombs.

South Korea on Thursday confirmed it would implement the latest U.N. sanctions imposed on North Korea, despite dire warnings from Pyongyang about the consequences.

The sanctions were ordered in response to the North's long-range rocket launch in December and prompted Pyongyang into threatening an imminent nuclear test.

A picture of North Korea's Kim Jong-Un with a smartphone has triggered fevered speculation about which brand is favoured by the leader of one of the world's most repressive nations.

The photo released by the North's state media showed Kim presiding over a meeting with top national security advisers last week -- a meeting believed to have been focused on Pyongyang's threat to conduct an imminent nuclear test.

South Korean President Lee Myung-Bak believes North Korea could detonate multiple devices when it goes ahead with a nuclear test expected in the coming weeks or even days.

In an interview published Tuesday in the Chosun Ilbo daily, the outgoing president also acknowledged the huge challenge the international community faces in seeking to wean Pyongyang off its nuclear weapons program.

The U.N. Security Council will take "very firm and strong" action against any nuclear test by North Korea, the council president said Tuesday.

Kim Sook, South Korea's U.N. envoy, said the test appeared "imminent" and any blast would be "a dangerous attempt to undermine the authority and credibility of the Security Council."

A defense ministry spokesman confirmed the three-day drill -- condemned as a "warmongering" exercise by North Korea -- was underway in the East Sea (Sea of Japan) off the southeastern South Korean port of Pohang.

Although South Korean military officials stressed the drill was scheduled before the North threatened to detonate its third nuclear device, the presence of the submarine has been seen as a warning to Pyongyang.

North Korean leader Kim Jong-Un has chaired a high-level meeting that discussed a looming "great turn" in military capability, state media said Sunday, fueling expectations of an imminent nuclear test.

Kim made a "historic" speech at the ruling party's Central Military Commission meeting, attended by the heads of the army, the National Defense Commission and the strategic rocket force, the official Korean Central News Agency (KCNA) reported.