The rocket launched by North Korea at the weekend seemed more powerful than its 2012 predecessor, but Pyongyang still lacks the expertise to transform it into a ballistic missile capable of reaching the US mainland, South Korean officials said Tuesday.

The comments came as leaders of South Korea, the United States and Japan discussed how to punish the North for its latest defiant launch and nuclear test, eyeing "strong and effective" UN sanctions.

The rocket, carrying an Earth observation satellite, blasted off on Sunday morning and, according to North Korean state TV, achieved orbit within 10 minutes.

The launch, which violated multiple UN resolutions, was widely seen as an act of open defiance just weeks after Pyongyang carried out its fourth nuclear test.

It sparked strong international condemnations and resulted in an agreement at the UN Security Council to move quickly to impose new sanctions.

The Pentagon said it wanted to send a sophisticated missile defence system to South Korea and that the two sides would start formal discussions on placing the Terminal High Altitude Area Defense System (THAAD) on the North's doorstep.

"Without getting into a timeline, we'd like to see this move as quickly as possible," Pentagon spokesman Peter Cook said.

A South Korean defence ministry official said on condition of anonymity that the latest rocket was similar to the Unha-3 launched in December 2012 but was believed to have an enhanced range of some 12,000 kilometres (7,500 miles).

The older version of the rocket had an estimated range of some 10,000 kilometres.

He was quick to emphasise, however, that the North has yet to master key technology needed to turn the rocket into an inter-continental ballistic missile, which would require a re-entry vehicle to protect the warhead from heat.

He added that the three-stage rocket was confirmed to have put an object into orbit but officials had yet to verify whether the purported satellite was functioning.

- 'Strong and effective sanctions' -

South Korean President Park Geun-Hye meanwhile spoke to US counterpart Barack Obama as part of a flurry of diplomatic activity aimed at agreeing on how best to censure the North.

Park and Obama agreed to cooperate "to make sure that the UN Security Council can adopt a resolution for strong and effective sanctions", the presidential Blue House said.

In addition to the UN measures, they agreed to hit the North with "various sanctions and pressure".

Park held a similar conversation with Japanese Prime Minister Shinzo Abe, who also spoke to Obama, telling him that Tokyo was considering its own sanctions against the North, Jiji Press said.

A draft sanctions resolution prepared by Japan, South Korea and the United States has been in negotiations for weeks, but veto-wielding China, the North's key ally, has been reluctant to back measures that would take aim at North Korea's already weak economy.

China fears that pushing Pyongyang too far could trigger instability that unleashes a wave of refugees flooding across its border.

Beijing also worries that a wholesale collapse of the regime in Pyongyang could lead to a US-allied unified Korea right on its doorstep.

The flight path of Sunday's rocket was similar to the 2012 launch vehicle, whose first stage debris was recovered by South Korea off its western coast, the defence ministry official said.

However, North Korea this time is believed to have had the first stage of Sunday's rocket blow up into around 270 pieces to cover up its technical footprint, he said.

The North is already subject to numerous UN sanctions over previous rocket launches and three nuclear tests in 2006, 2009 and 2013.

It routinely insists the launches are part of a legitimate space exploration programme but the United States and its allies view them as disguised ballistic missile tests.