SEOUL, April 23 (Reuters) - North Korea fired a submarine-launched ballistic missile on Saturday off its east coast, South Korea said, amid concerns that the isolated state might conduct a nuclear test or a missile launch ahead of a ruling party meeting in May.

The North fired the missile to the northeast from an area off its east coast at about 6:30 p.m. (0930 GMT), the South's office of the Joint Chiefs of Staff said.

North Korea will hold a rare congress of its ruling Workers' Party in early May for the first time in 36 years where its leader Kim Jong Un is expected to proclaim the country was a strong military power and a nuclear state.

The missile flew for about 30 km (18 miles), a South Korean Defence Ministry official said by telephone, adding its military was trying to determine whether the launch may have been a failure for unspecified reasons.

Click through for photos of North Korea's nuclear facilities:



13 PHOTOS North Korean nuclear facilities See Gallery North Korea fires submarine-launched missile - South Korea YONGBYON NUCLEAR FACILITY, NORTH KOREA - JULY 2, 2015: Figure 3. Transformer Yard at the ELWR. Date: July 2, 2015. Yongbyon Nuclear Scientific Research Center. Mandatory credit: Image Â© 2015 DigitalGlobe Inc./38 North. YONGBYON NUCLEAR FACILITY, NORTH KOREA - JULY 2, 2015: Figure 4. Construction at the Uranium Enrichment Facility. Date: July 2, 2015. Yongbyon Nuclear Scientific Research Center. Mandatory credit: Image Â© 2015 DigitalGlobe Inc./38 North. YONGBYON NUCLEAR FACILITY, NORTH KOREA - JULY 2, 2015: Figure 1. Vehicles at the 5 MWe Reactor. Date: July 2, 2015. Yongbyon Nuclear Scientific Research Center. Mandatory credit: Image Â© 2015 DigitalGlobe Inc./38 North. YONGBYON NUCLEAR FACILITY, NORTH KOREA - APRIL 29, 2015: Figure 2. Probable CO2 truck seen at the 5 MWe Reactor in April. Date April 29, 2015. Yongbyon Nuclear Scientific Research Center. Mandatory credit: Image Â© 2015 DigitalGlobe Inc./38 North. YONGBYON NUCLEAR FACILITY, NORTH KOREA - May 24, 2015: Figure 4. Continued construction at the uranium enrichment facility. Note: image rotated. Date: May 24, 2015. Published on 38 North. (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - NOVEMBER 4, 2014: This is Figure 3 -- DigitalGlobe imagery shows new activity seen at the Radiochemical Laboratory. Note: image rotated -- published on 38 North. (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - JANUARY 11, 2015: This is Figure 4B. ELWR activity in January 2015. Note: image rotated. Date: January 11, 2015 -- published on 38 North. (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - May 24, 2015: Figure 3: 5 MWe Reactor on May 24vehicles present, no indications of steam or water discharge. Note: image rotated. (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - May 24, 2015: Figure 2. Power line service to 5 MWe Reactor area. Note: image rotated. Date: May 24, 2015. Published on 38 North. (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - OCTOBER 28, 2014: This is Figure 1 -- DigitalGlobe imagery of the 5 MWe Reactor appears to still be shutdown -- published on 38 North. (Photo DigitalGlobe/38 North via Getty Images) PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 27th, 2015: Figure 6a. Construction material visible at the Main Support Area at the Punggye-ri Nuclear Test Site in North Korea. Date: March 27, 2015. (Photo DigitalGlobe/38 North via Getty Images) PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MAY 16, 2015: Figure 6b. An old building has been re-roofed and a new building has been erected at the Main Support Area at the Punggye-ri Nuclear Test Site in North Korea. Date: May 16, 2015 (Photo DigitalGlobe/38 North via Getty Images) YONGBYON NUCLEAR FACILITY, NORTH KOREA - FEBRUARY 7, 2013: This is Figure 9. a time progression of the experimental light water reactor (ELWR) development at the Yongbyon Nuclear Facility in North Korea. Featured in âStart-Up of North Korean Experimental Light Water Reactor Could Begin by Mid-2013 If Fuel is Available,â published on 38 North. Figure 9: February 7, 2013 (reactor building complete, cleanup underway). (Photo DigitalGlobe/38 North via Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

South Korea's Yonhap news agency said the missile flew "for a few minutes," citing a government source.

The North first attempted a launch of the submarine-based missile last year and was seen to be in the early stages of developing such a weapons system, which could pose a new threat to its neighbors and the United States if it is perfected.

However, follow-up test launches were believed to have fallen short of the North's expectations as its state media footage appeared to have been edited to fake success, according experts who have seen the visuals.

South Korea's military has said it is on high alert over the possibility that the isolated North could conduct its fifth nuclear test "at any time" in defiance of U.N. sanctions after setting off what it said was a hydrogen device in January.

Satellite images show that North Korea may have resumed tunnel excavation at its main nuclear test site, similar to activity seen before the January test, a U.S. North Korea monitoring website reported on Wednesday.

South Korea and the United States, as well as experts, believe the North is working to develop a submarine-launched ballistic missile system and an intercontinental ballistic missile (ICBM) putting the mainland United States within range.

North Korea is banned from nuclear tests and activities that use ballistic missile technology under U.N. sanctions dating back to 2006 and most recently adopted in March but it has pushed ahead with work to miniaturize a nuclear warhead and develop an ICBM.

A senior U.S. official said this week that North Korea should take a lesson from Iran which has agreed to roll back its nuclear program in an agreement with Western powers in return for lifting of major sanctions but the North has shown no sign of entering into such a pact. (Editing by Jack Kim and Alison Williams)

Take a look at daily life in North Korea:



23 PHOTOS Take a look inside North Korean daily life See Gallery North Korea fires submarine-launched missile - South Korea General view of the capital with the Golf club near the river Taedong ,August 10,2005,in Pyongyang ,North Korea. (Photo by Patrick Aventurier/Getty Images) North Korean leader Kim Jong Un speaks during the first party committee meeting in Pyongyang, in this undated photo released by North Korea's Korean Central News Agency (KCNA) December 25, 2016. (KCNA KCNA / Reuters) A general view of downtown street on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) North Korean children practise table tennis against the wall in a primary school on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) The Monument to the Party Founding is seen on April 3, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) Cyclists ride along the banks of the Taedong River in front of the Grand People's Study House opposite the Tower of the Juche Idea. The 100,000 square metre building can house 30 million volumes and accommodate over 12,000 peeople daily; it contains 600 rooms. (Photo by Alain Nogues/Alain Nogues/Corbis via Getty Images) North korean pioneers going to pay respect to the dear leaders at mansudae art studio, pyongyang, North Korea on September 8, 2012 in Pyongyang, North Korea. (Eric Lafforgue/Art in All of Us via Getty Images) People commute on the bus near April 25 House of Culture, the venue of the Workers' Party of Korea (WPK) congress in Pyongyang, North Korea May 6, 2016. (REUTERS/Damir Sagolj) Commuters queue for buses on October 30, 2014 in Pyongyang, North Korea. (Photo by The Asahi Shimbun via Getty Images) A man looks across Yalu River towards North Korea as he stands next to the bridge that connects China's Dandong, Liaoning province and North Korea's Sinuiju, September 10, 2016. (REUTERS/Thomas Peter/File Photo) A soldier walks under portraits of North Korea's founder Kim Il-sung (L) and former leader Kim Jong-il at Pyongyang's main square October 11, 2015. Isolated North Korea marked the 70th anniversary of its ruling Workers' Party on Saturday with a massive military parade overseen by leader Kim Jong Un, who said his country was ready to fight any war waged by the United States. (REUTERS/Damir Sagolj) O Yang Ran and her husband Kim Chol Nam pose during a photo session at Pyongyang Folk Park, October 11, 2015. Isolated North Korea marked the 70th anniversary of its ruling Workers' Party on Saturday with a massive military parade overseen by leader Kim Jong Un, who said his country was ready to fight any war waged by the United States. (REUTERS/Damir Sagolj) A boy dressed as solider wait for the bus with others in central Pyongyang, October 11, 2015. Isolated North Korea marked the 70th anniversary of its ruling Workers' Party on Saturday with a massive military parade overseen by leader Kim Jong Un, who said his country was ready to fight any war waged by the United States. (REUTERS/Damir Sagolj) A Chinese man washes his mop on February 14, 2012 in Dandong, China. Dandong lies on the border between China and North Korea and is the largest border city in China. (Photo by Hong Wu/Getty Images) A man rides a bicycle in a park on April 3, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) Two North Korean boys make a fun on the street on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) Commuters move by escalator at Puhung Subway station situated more than 100M below the surface, it is also an atomic shelter, on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) North Korean children play football in a primary school on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) North Korean children perform in a primary school on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) North Korean children perform in a primary school on April 2, 2011 in Pyongyang, North Korea. Pyongyang is the capital city of North Korea and the population is about 2,500,000. (Photo by Feng Li/Getty Images) The Immortal Statue of Kim Il Sung monument is seen on August 12, 2009 in Pyongyang, North Korea. (Photo by NK News/Getty Images) Portraits of President Kim Il Sung (L) and his son Kim Jong Il, current Secretary General of the Korean Workers Party and Chief General of the army, in the international conference room for Korea's reunification. (Photo by Alain Nogues/Corbis via Getty Images) This photo taken on January 7, 2017 and released on January 9, 2017 by North Korea's official Korean Central News Agency (KCNA) shows a joint conference of officials of the party, state, economic and armed forces organs to discuss ways for carrying through the tasks set forth by North Korean leader Kim Jong-Un in his New Year address in Pyongyang. (STR/AFP/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

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