North Korea on Thursday conducted what appeared to be a failed test -- its second in a week -- of a powerful medium-range missile that experts warn could be deployed as early as next year.

South Korean and US military monitors said the missile -- believed to be an intermediate-range Musudan -- exploded shortly after take-off at around 6:30 am Pyongyang time (2200 GMT Wednesday).

The attempted launch came just hours before the start of the third US presidential debate -- a timely reminder of the challenge North Korea's fast-moving nuclear weapons programme will pose to the next occupant of the White House.

It also followed a meeting in Washington between the US and South Korean defence and foreign ministers, at which US Secretary of State John Kerry stressed that any use of nuclear weapons by the North would be "met with an effective and overwhelming response".

It was the second failed launch in less than a week of the Musudan, which has a theoretical range of anywhere between 2,500 and 4,000 kilometres (1,500 and 2,500 miles).

The lower estimate covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.

- US anger -

The US and its two key Asian allies all condemned the latest launch as a clear violation of UN resolutions banning the North from using ballistic missile technology.

"Our commitment to the defence of our allies, including the Republic of Korea and Japan, in the face of these threats, is ironclad," said Pentagon spokesman Gary Ross.

South Korean President Park Geun-Hye said the North was "blatantly threatening" the entire region, while her foreign ministry denounced Pyongyang's "manic obsession" with nuclear bombs and missile.

The last Musudan test on Saturday was denounced by the UN Security Council, which is currently debating a fresh sanctions resolution against Pyongyang over its fifth nuclear test carried out last month

The missile has now been tested eight times this year -- but only once successfully.

A Musudan launched in June flew 400 kilometres into the Sea of Japan (East Sea), and was hailed by leader Kim Jong-Un as proof of the North's ability to strike US bases across "the Pacific operation theatre".

Despite the string of failures, some experts believe the missile is moving swiftly towards operational deployment.

- Fast-track testing -

According to John Schilling, an aerospace engineer specialising in rocket propulsion, the aggressive launch schedule, while multiplying the risk of failure, also increases the information gleaned from each test.

"If they continue at this rate, the Musudan intermediate-range ballistic missile could enter operational service sometime next year -- much sooner than had previously been expected," Schilling wrote recently on the 38North website of the US-Korea Institute at Johns Hopkins University.

North Korea has been hit by five sets of UN sanctions since it first tested a nuclear device in 2006.

After Pyongyang carried out its fourth nuclear test in January, the Security Council adopted the toughest sanctions resolution to date, targeting North Korea's trade in minerals and tightening banking restrictions.

The ongoing negotiations on the new sanctions measure are focused on closing loopholes and zeroing in on North Korea's nuclear and ballistic missile technology industry, according to Security Council diplomats.