A magnitude 5.1 earthquake located off the west coast of South Korea has occurred just days after North Korea threatened to conduct a “new form” of nuclear test, the United States Geological Survey (USGS) has said.

The quake centered about 100 km north-west of the West Gyeongnyeolbi Archipelago, off Taean County, and was the third strongest in or around the Korean peninsula since official seismological records began in 1978.

On Sunday, North Korea warned it “would not rule out a new form of nuclear test for bolstering up our nuclear deterrence,” but did not give any further details on what this would entail, in response to United Nations Security Council's condemnation of the country’s recent ballistic missile launch.

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The earthquake took place at a depth of nearly 16km (10 miles) in the sea west of the Korean peninsula, which did not immediately suggest North Korean nuclear testing was the cause, according to The Telegraph.

Meanwhile, an unmanned drone crashed on a South Korean island near a disputed maritime border with North Korea, a South Korean defence ministry official said on Tuesday, triggering an investigation into whether the aircraft was from the North.

Shape Created with Sketch. In pictures: South and North Korea exchange fire as tensions rise with US Marines' exercise Show all 12 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. In pictures: South and North Korea exchange fire as tensions rise with US Marines' exercise 1/12 South Korea A South Korean marine LVT-7 landing craft sail to shores through a smoke screen during the U.S.-South Korea joint landing exercises called Ssangyong, part of the Foal Eagle military exercises in Pohang 2/12 South Korea Amphibious assault vehicles of the South Korean Marine Corps throw smoke bombs as they move to land on shore during a U.S.-South Korea joint landing operation drill in Pohang 3/12 South Korea The planned North Korean drills come after an increase in threatening rhetoric from Pyongyang and a series of rocket and ballistic missile launches in an apparent protest against the annual military exercises by Seoul and Washington 4/12 South Korea South Korean Marine amphibious assault vehicles land on the seashore during a joint landing operation by U.S. and South Korean Marines in Pohang 5/12 South Korea A South Korean Marine stands on a beach as amphibious assault vehicles (in background) approach the seashore during a joint landing operation by U.S. and South Korean Marines in Pohang 6/12 South Korea U.S. Marines take a position during a joint landing operation by US and South Korean Marines in Pohang. North Korea announced a live-fire drill near its disputed maritime border with South Korea, further ratcheting up tensions a day after threatening a 'new form' of nuclear test 7/12 South Korea Marines from 3rd Marine Expeditionary Force, Battalion landing team deployed from Okinawa, Japan participate in the U.S. and South Korean Marines joint landing operation at Pohang seashore in Pohang. It has been reported by South Korean media that the scale of this annual joint exercise is one of the largest in the past 8/12 South Korea U.S. marines participate in a U.S.-South Korea joint landing operation drill in Pohang 9/12 South Korea The drill is part of the two countries' annual military training called Foal Eagle, which runs until April. North Korea declared a no-sail warning for areas off its west coast near a disputed border with South Korea and has notified the South that it will conduct firing drills 10/12 South Korea The warning from North Korea comes amid heightened tensions surrounding the North after the U.N. Security Council condemned Pyongyang for its mid-range missile launches, just as the leaders of South Korea, Japan and the United States met to discuss the North's arms programme 11/12 South Korea An amphibious assault vehicle of the South Korean Marine Corps arrives on shore during a U.S.-South Korea joint landing operation drill in Pohang 12/12 South Korea South Korean marine's Landing Vehicle Tracked (LVT) move their position during the U.S. and South Korean Marines joint landing operation at Pohang seashore in Pohang 1/12 South Korea A South Korean marine LVT-7 landing craft sail to shores through a smoke screen during the U.S.-South Korea joint landing exercises called Ssangyong, part of the Foal Eagle military exercises in Pohang 2/12 South Korea Amphibious assault vehicles of the South Korean Marine Corps throw smoke bombs as they move to land on shore during a U.S.-South Korea joint landing operation drill in Pohang 3/12 South Korea The planned North Korean drills come after an increase in threatening rhetoric from Pyongyang and a series of rocket and ballistic missile launches in an apparent protest against the annual military exercises by Seoul and Washington 4/12 South Korea South Korean Marine amphibious assault vehicles land on the seashore during a joint landing operation by U.S. and South Korean Marines in Pohang 5/12 South Korea A South Korean Marine stands on a beach as amphibious assault vehicles (in background) approach the seashore during a joint landing operation by U.S. and South Korean Marines in Pohang 6/12 South Korea U.S. Marines take a position during a joint landing operation by US and South Korean Marines in Pohang. North Korea announced a live-fire drill near its disputed maritime border with South Korea, further ratcheting up tensions a day after threatening a 'new form' of nuclear test 7/12 South Korea Marines from 3rd Marine Expeditionary Force, Battalion landing team deployed from Okinawa, Japan participate in the U.S. and South Korean Marines joint landing operation at Pohang seashore in Pohang. It has been reported by South Korean media that the scale of this annual joint exercise is one of the largest in the past 8/12 South Korea U.S. marines participate in a U.S.-South Korea joint landing operation drill in Pohang 9/12 South Korea The drill is part of the two countries' annual military training called Foal Eagle, which runs until April. North Korea declared a no-sail warning for areas off its west coast near a disputed border with South Korea and has notified the South that it will conduct firing drills 10/12 South Korea The warning from North Korea comes amid heightened tensions surrounding the North after the U.N. Security Council condemned Pyongyang for its mid-range missile launches, just as the leaders of South Korea, Japan and the United States met to discuss the North's arms programme 11/12 South Korea An amphibious assault vehicle of the South Korean Marine Corps arrives on shore during a U.S.-South Korea joint landing operation drill in Pohang 12/12 South Korea South Korean marine's Landing Vehicle Tracked (LVT) move their position during the U.S. and South Korean Marines joint landing operation at Pohang seashore in Pohang

The South Korean military was trying to verify where the drone had come from and what its purpose might have been, and was also looking into any possible link to North Korea's espionage operations, a military official told Reuters.

On Monday, North and South Korea fired artillery shells into each other's waters, forcing people on five South Korean islands on the frontline to evacuate their homes.

Early that morning, North Korea announced that it would conduct live-fire drills in seven areas north of the Yellow Sea boundary between the nations.

South Korea responded by returning fire after North Korean shells landed in its territorial waters.