The UN Security Council on Friday strongly condemned North Korea over its ballistic missile launches and demanded Pyongyang to refrain from further violations of UN resolutions.

Backed by China, Pyongyang's ally, the council said in a unanimous statement that "all these launches were unacceptable" and "constituted a clear violation of UN Security Council resolutions."

The statement was adopted during a closed-door meeting called by the United States after North Korea test-fired two medium-range ballistic missiles, the latest in a string of provocative acts from the reclusive regime.

Council members "strongly condemned and expressed grave concern at the ballistic missile launches" and declared that North Korea "shall refrain from further actions in violation" of UN resolutions.

Two weeks ago, the Security Council imposed its toughest sanctions to date on North Korea after Pyongyang carried out its fourth nuclear test and fired a rocket that was widely seen as a disguised ballistic missile test.

US Ambassador Samantha Power said the latest missile launches underscored the importance of implementing the new sanctions resolution, which targets North Korea's mining, trade and financial sectors.

"If anybody on the council needed a reminder of why that resolution is so important (...) the North Korean regime just provided another one," Power said.

The launches came a day after US President Barack Obama signed an order implementing the tough sanctions outlined in the recent UN resolution, as well as new unilateral US measures.

Japan's UN Ambassador Motohide Yoshikawa called the latest missile launches "very, very unfortunate" and said Pyongyang had not received "the message" from the council.

British Deputy UN Ambassador Peter Wilson said "this is exactly the sort of thing that they should not be doing."

"What we see yet again is the North Koreans defying the will of the international community and the Security Council," he said.

- North Korea escalates -

UN Secretary-General Ban Ki-moon said the firing of the two missiles was "deeply troubling" and urged Pyongyang to halt "these inflammatory and escalatory actions," his spokesman said.

Ban called on North Korea to comply with UN resolutions that bar the country from developing missile technology.

During remarks at an event with North Korean women at the US mission, Power took an apparent swipe at China, saying it would be "absurd" to disassociate North Korea's dismal rights situation from its military ambitions.

"Many of North Korea's systematic human rights violations deliberately underwrite the government's nuclear program, including the forced labor carried out by tens of thousands of women and children," said Power.

China has opposed discussion in the Security Council of North Korea's rights record, arguing that the forum for this was the Human Rights Council in Geneva.

US Defense Secretary Ashton Carter said China, Pyongyang's main trading partner and benefactor, could "do a lot more" to get North Korea to change course.

North Korean leader Kim Jong-Un this week ordered multiple ballistic missile launches and a nuclear warhead test, sparking fresh concerns about the regime's intransigeance.

North Korea could carry out another nuclear test at any time, a US think tank has suggested, after analysing satellite imagery from Pyongyang's main testing site.

Activity at the Punggye-ri underground facility suggested the North was maintaining tunnels as well as cleaning up after its detonation in January, the US-Korea Institute at Johns Hopkins University said on Friday.

"It is highly likely that (the) site is capable of supporting additional tests at any time," it said on its closely watched 38North website.

South Korean military officials said two missiles were launched on Friday from Sukchon in the country's southwest.

US defence officials said they believed Pyongyang fired medium-range Rodong missiles from road-mobile launch vehicles.

The Rodong is a scaled-up Scud variant with a maximum range of around 1,300 kilometres (800 miles).

A Rodong test is more provocative, given its greater range, which makes it capable of hitting most of Japan.

South Korea's defence ministry said Friday's launches were clearly the result of Kim's order.

"North Korea appears to be speeding up test launches to advance its nuclear capabilities," said ministry spokesman Moon Sang-Gyun.

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