Four missiles were fired on Monday (March 6) from North Pyongan Province into the East Sea, also known as the Sea of Japan.
The missiles were unlikely to have been intercontinental ballistic missiles (ICBM) which could reach the United States, but were said to have travelled around 1,000km and reached an altitude of 260km.
Three of the missiles went down in Japan's Exclusive Economic Zone (EEZ) - waters extending 200 nautical miles or 370km from its coast.
Japanese Prime Minister Shinzo Abe said: "This clearly shows North Korea has entered a new stage of threat."
We take stock of North Korea's missile and nuclear development.
1. How advanced is its missile programme?
After North Korea set off its most powerful nuclear blast to date on Sept 9, 2016, it said it had mastered the ability to mount a warhead on a ballistic missile.
South Korean President Park Geun Hye had said three days after the North's test: "Its nuclear-tipped missile is no longer a mere threat or negotiating tool, but an actual urgent threat facing us."
North Korea is believed to possess over 1,000 missiles of varying capabilities.
Since late 1970s, when North Korea started developing a version of the Soviet Scud-B, the country has progressed to multi-stage missiles, and is believed to be working on ICBMs with a range of greater than 5,500km which could have the capability to hit targets in the US mainland.
The North announced in April 2016 that it had tested a new type of ICBM engine.
It is now working on re-entry technology for ICBMs to ensure a warhead could re-enter the Earth's atmosphere without breaking up.
Professor Kim Dong Yup at Kyungnam University's Far East Institute had told news website NK News that Pyongyang already has re-entry technology for short-range missiles.
The intelligence community estimates that North Korea can achieve re-entry capability in three to five years.
2. How advanced is its nuclear weapon programme?
North Korea has since 2006 conducted five nuclear tests. Last year alone, it conducted two within the space of eight months.
Analyst Jeffrey Lewis at the Center for Nonproliferation Studies at the Middlebury Institute of International Studies in Monterey, California, after studying a March 2016 picture of North Korean leader Kim Jong Un unveiling what was claimed to be a new nuclear device, estimated the device has a destructive yield of about 20 kilotonnes.
That is roughly equivalent to the atomic bombs that the US dropped on Japan during World War II, the New York Times reported.
More significantly, the device's small size, about 60cm in diameter, seems to prove North Korea's claim that it can fit nuclear warheads on its long-range missiles.
Such a major leap forward for the North's nuclear prowess is undoubtedly a threat to countries such as South Korea, Japan and the US.
Professor Siegfried S. Hecker of Stanford University said in September 2016 that Pyongyang's ability to launch an ICBM fitted with a nuclear warhead capable of reaching the US was "still a long way off - perhaps five to 10 years, but likely doable if the programme is unconstrained", BBC reported.
North Korea had also claimed that its January 2016 test was of a hydrogen bomb, which would be more powerful than atomic bombs used in the previous three tests.
It is also believed the North is seeking to use uranium as the starting material for its nuclear tests. While North Korea's plutonium stocks are finite, it could build up a nuclear stockpile by enriching uranium. The North has abundant reserves of uranium ore.
3. Where does the money to develop missile and nuclear programmes come from?
While commoners in North Korea may go without the basic day-to-day needs, including electricity and even food, Pyongyang continues to pour about 25 per cent of its Gross Domestic Product on military spending.
Its GDP was reported to be worth US$16.12 billion (S$22.7 billion) in 2015.
The country earns money through its tourism business, exporting minerals and metallurgical products, as well as through illicit activities of trading armaments and military equipment.
It was reported that about 90 per cent of North Korea's trade, worth about US$6 billion a year, is with China.
Beijing, in a bid to impose United Nations Security Council sanctions aimed at stopping Pyongyang's nuclear weapon and missile programmes, has suspended since Feb 19 all imports of coal from the North.
Still, North Korea has so far been able to evade international sanctions with a sophisticated network of overseas companies, according to a forthcoming UN report. The North has enjoyed continued access to the international banking system.
According to the CIA World Factbook, North Koreans are also involved in drug trafficking, supplying heroin and methamphetamine.
Chronology of recent missile and nuclear developments:
May 15, 2011
North Korea and Iran are suspected of sharing ballistic missile technology, according to a UN sanctions report.
April 13, 2012
A long-range Unha-3 rocket is launched from the Sohae satellite base but disintegrates soon after blast-off and falls into the ocean.
Dec 12, 2012
North Korea launches a three-stage, Unha-3 satellite rocket from the Sohae site. But foreign experts say the satellite has never functioned, giving rise to suspicion that it is actually a banned ballistic missile test. The basic technology applied in satellite launches and ballistic missile development is similar.
Feb 12, 2013
North Korea conducts its third underground nuclear test. Previous nuclear tests were conducted in 2009 and 2006.
Jan 6, 2016
North Korea conducts its fourth underground nuclear test, which it says is of a hydrogen bomb, a claim doubted by most experts.
Feb 7, 2016
North Korea says it has placed another Earth observation satellite in orbit with its second successful space rocket launch
March 9, 2016
North Korea announces that it has miniature nuclear warheads that can be mounted on ballistic missiles.
Sept 9, 2016
North Korea claims to have detonated a nuclear warhead in its fifth nuclear test. The blast is estimated to have the explosive power of between 10 and 30 kilotonnes.
Feb 12, 2017
North Korea test-fires a ballistic missile. It travels nearly 500km from North Pyongan Province to the East Sea, or Sea of Japan.
March 6, 2017
North Korea fires four ballistic missiles, three of them fall into Japan’s exclusive economic zone.
March 19, 2017
North Korea announces rocket engine test, saying it will help country achieve “world-class satellite launch capability.”
March 22, 2017
A North Korean missile appears to explode just after launch.
April 5, 2017
North Korea fires a ballistic missile into the sea off its east coast. US officials say it appears to be a liquid-fueled, extended-range Scud missile.
SOURCES: Agence France-Presse, Reuters, BBC, CNN, The Economist, InsideGov, New York Times