The UN Security Council president condemned nuclear-armed North Korea over its tests of a powerful new medium-range missile on Wednesday, calling for a swift response from the world body.

Francois Delattre of France said the back-to-back tests were a "clear and unacceptable" violation of Security Council resolutions.

South Korea's defense ministry said the two missiles achieved a significant increase in flight distance over previous failed launches and were believed to be of a much-hyped, intermediate-range Musudan missile -- theoretically capable of reaching US bases as far away as Guam.

Delattre called for a swift response by the UN and said the Security Council would likely meet later in the day.

"We favor a quick and firm reaction of the Security Council," he said.

"The North Korean ballistic program is a serious threat to regional and international peace and security," he added.

"Confronted by the threat of proliferation we consider that weakness is not an option."

Condemnation was swift from the United States, NATO and Japan, with South Korea vowing to push for tighter sanctions on Pyongyang.

The first test was deemed to have failed after the missile flew an estimated 150 kilometers (90 miles) over the East Sea, or Sea of Japan.

Japanese military monitors said the second test attained a height of 1,000 kilometers and a range of 400 kilometers -- a trajectory some experts suggested was calculated to avoid any violation of Japanese air space.

Four previous Musudan tests this year failed either on their mobile launch pad or shortly after take-off.

Existing UN Security Council measures ban North Korea from any use of ballistic missile technology.

After Pyongyang conducted a fourth nuclear test on January 6, followed by a long-range rocket launch February 7, the Security Council adopted its most punishing sanctions yet against North Korea.

- Worrying progress -

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.

Melissa Hanham, an expert on North Korea's WMD program at the Middlebury Institute of International Studies in California, said Wednesday's launches represented a worrying step forward.

"The second was likely a success. Testing is iterative and they are learning from each flight," Hanham told AFP.

"Policymakers need to focus on a testing ban to prevent this from becoming a working missile."

US 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", while NATO "strongly condemned" the launch in a statement from its secretary general.

South Korea's foreign ministry warned that North Korea would face even stronger sanctions and said the tests underlined "the hypocrisy and deceptiveness" of Pyongyang's recent offers of military talks with Seoul.

China, traditionally the North's closest ally, cautioned against "any action that may escalate tension" and called for a resumed dialogue on Pyongyang's nuclear drive.

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 kilometers.

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

- String of failures -

Three failed launches in April were seen as an embarrassment for North Korea's leadership, coming ahead of a rare ruling party congress that was meant to celebrate the country's achievements.

Another attempt in May was also deemed to have failed.

Markus Schiller, a German aerospace engineer who has written extensively on North Korea's missile program, said a lot of information on the Musudan was highly speculative and warned against drawing too many conclusions from Wednesday's launches.

"We do not even know if these were indeed Musudan missiles that were launched today," Schiller said.

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