North Korea has upset the world and regional powers again - this time, with a fourth nuclear bomb test and long-range missile launch, within weeks of each other.

On Feb 7, North Korea fired a "rocket" carrying a satellite, which critics see as a disguise for an intercontinental ballistic missile (ICBM) test.

This interpretation does not come without precedent. After the August 1998 "rocket" launch, North Korea's official daily Rodong Sinmun stated that world opinion evaluated that the development of its satellite launch vehicle was equivalent to the "development of an ICBM", suggesting Pyongyang understood the military implications of launching such a rocket.

The recent launch repeated and added to the success of its former launch in December 2012, with the ability to carry a heavier satellite, twice the weight of its predecessor, and a longer flight range of reportedly up to 12,000km - 2,000km more than its previous range.

With a payload of 500kg, North Korea now has the ability to hit a large part of the United States, including Washington DC and New York.

Despite its technological advances, however, the threat of an actual nuclear strike against the US mainland is not yet realistic. Other crucial elements remain unfulfilled, including a re-entry test proving that its ICBM warhead has a heat shield, and the replacement of its open launch sites with hardened silos to withstand pre-emptive strikes.

North Korea also showed advances in its nuclear bomb test on Jan 6. While the explosion yield has plateaued to several kilotons, similar to that in the third nuclear test in 2013, its reliability has certainly improved with two consecutive successes.

After the 2013 test, the US Defence Intelligence Agency assessed that the reliability of North Korea's nuclear weapons was "low". After the recent test, it might have to change that assessment.

Moreover, while the limited explosion yield would not be capable of destroying hardened military targets, it would be good enough to demolish unhardened military facilities and create havoc in soft targets such as major cities. It can also mean that it is more usable, as the level of damage can be controlled.

North Korea has conducted both nuclear tests and long-range missile tests with two- to three-year intervals: the first runs in 2006, the second runs in 2009, and the third runs in 2012 and 2013. The recent tests fit in this pattern, and are technologically sound.

MESSAGE TO CHINA

While North Korea provoked its neighbours and the US, there seems to be an implicit message to China - to acknowledge North Korea as a significant country and Mr Kim Jong Un as a strong leader.

In October last year, China sent Mr Liu Yunshan - a member of the Communist Party's most powerful Politburo Standing Committee - to Pyongyang, in order to improve its relations with North Korea. An implicit message was that a China-North Korea summit meeting would be possible if Mr Kim behaved properly, that is, decide not to conduct further nuclear and missile tests.

In December, however, Mr Kim responded to this by announcing that his country possessed atomic and hydrogen bombs. In the same period, Mr Kim sent North Korea's all-female music group, which he had formed in 2012, to Beijing. But then, it was revealed that the band was preparing a performance that in part celebrated North Korea's nuclear and missile development. China refused to have high-ranking Communist Party officials attend the performance. To this, Mr Kim decided to cancel the performance and bring the band home. Mr Kim's message was clear: If China wanted to improve its relations with North Korea, it must first recognise his country as a nuclear-weapon state.

North Korea also seems to have further strained China's relations with the US, South Korea and Japan, as its recent tests are likely to spur US defence build-up in Asia. With the escalating threats of North Korea's missile technology, South Korea has become more willing to consider US deployment of the Terminal High-Altitude Area Defence missile defence system in South Korea. Beijing would regard such a deployment as a first step towards the establishment of a US-South Korea-Japan coalition against China.

Moreover, the new sanctions the US decided to impose on North Korea last week can affect Chinese banks and business enterprises. In fact, these sanctions were designed to pressure Beijing to pressure Pyongyang more strongly.

DOMESTIC OBJECTIVES

North Korea's recent provocations also have domestic implications. It is likely that Mr Kim desired to demonstrate advances in his missile and nuclear programmes in the lead-up to the Feb 16 birthday of his late father. In fact, the missile carried Mr Kim Jong Il's pseudonym - "Kwangmyongsong" or shining star - on its body.

Mr Kim Jong Un also seeks to showcase his strength internally before the Seventh Workers' Party Congress in May. This top-level party congress will be the first one in 36 years. The last convention, which was also the sixth, took place in 1980 and was held under the country's founder, Kim Il Sung.

The party congress is of symbolic significance, as Mr Kim Jong Il made his political debut at the Sixth Party Congress, confirming him as successor. Previous conventions have also been used to discuss major state policies, elect top party officials, and adopt party regulations. The upcoming congress will likely mark the advent of a new era under Mr Kim Jong Un, possibly with an important policy shift and leadership reshuffle.

WHAT'S NEXT?

In response to North Korea's recent tests, South Korea has suspended operations at its joint Kaesong factory complex for the first time since it opened in 2004. The move sends an important signal as Kaesong was one of the few remaining examples of cooperation between the two Koreas. Inter-Korean relations are expected to remain tense, overshadowing Seoul's initiatives for unification.

Japan has tightened sanctions on North Korea, leading to Pyongyang's suspension of previously negotiated investigations into the abductions of Japanese citizens. The US Senate also approved more sanctions, and the US and China are negotiating the outline of a new United Nations sanctions resolution.

In particular, Washington is pressing Beijing to accept the restriction of North Korean access to international ports. Given that a large portion of China's trade with North Korea is achieved through ports in North-east China, the outcome of the negotiation will reveal China's true intentions.

The critical questions are whether China will agree to put real pressure on Mr Kim; and what kind of policy change North Korea's young leader will introduce in May. Answers will emerge soon.

•The writer is professor at the National Graduate Institute for Policy Studies in Tokyo and currently a visiting fellow at the Woodrow Wilson International Centre for Scholars in Washington DC.