Nuclear-armed North Korea conducted two back-to-back tests of a powerful new medium-range missile on Wednesday, South Korea’s Defense Ministry said.

Nuclear-armed North Korea conducted two back-to-back tests of a powerful new medium-range missile on Wednesday, with both achieving a significant increase in flight distance over previous failed launches, South Korea's Defense Ministry said.

Both tests were believed to be of a much-hyped, intermediate-range Musudan missile capable of reaching US bases as far away as Guam.

The first test shortly before 6:00 am (2100 GMT Tuesday) was deemed to have failed after reportedly flying around 150 kilometers (90 miles) over the East Sea (Sea of Japan).

The South Korean defense ministry said the second Musudan -- fired from the same east coast location two hours later -- had flown 400 kilometers.

"South Korea and the United States are conducting further analysis," the ministry said in a statement that stopped short of labeling the second test a success or failure.

Four failed Musudan tests earlier this year all either exploded on the mobile launch pad or shortly after take-off.

A successful test would mark a major step forward for a weapons program that ultimately aspires to develop a proven nuclear strike capability against the US mainland.

The US strongly condemned the launches, saying they represented clear violations of UN resolutions banning North Korea from any use of ballistic missile technology.

State Department spokesman John Kirby said the latest launches would only increase global efforts to counter North Korea's illicit weapons program.

"We intend to raise our concerns at the UN to bolster international resolve in holding (North Korea) accountable for these provocative actions," Kirby said in a statement.

Japanese broadcaster NHK quoted Prime Minister Shinzo Abe as saying such tests "cannot be tolerated".

A Pentagon statement said the US military had tracked both missiles and determined they "did not pose a threat to North America".

First unveiled as an indigenous missile at a military parade in Pyongyang in October 2010, the Musudan has a theoretical range of anywhere between 2,500 and 4,000 kilometres.

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