North Korea appears to be preparing to launch a long-range missile, according to South Korea's defence ministry and Japanese media.
Activity has been spotted at a launch station on the west coast of the isolated nation.
Earlier this week Pyongyang announced it was planning to launch a satellite at some point in February.
The announcement was internationally condemned - critics say it is a cover to test banned missile technology.
North Korea also conducted its fourth nuclear bomb test on 6 January.
UN sanctions against North Korea prohibit it from carrying out any nuclear or ballistic missile tests.
North Korea's rocket launches
- May 2015: North Korea announces it has successfully tested a submarine-launched missile for the first time, but scepticism is then poured on the claim
- Dec 2012: North Korea launches three-stage rocket, says it successfully put a satellite into orbit; US defence officials confirm object in orbit
- Apr 2012: Three-stage rocket explodes just after take-off, falls into sea
- Apr 2009: Three-stage rocket launched; North Korea says it was a success, US says it failed and fell into the sea
- Jul 2006: North Korea test-fires a long-range Taepodong-2 missile; US said it failed shortly after take-off
South Korean state news agency Yonhap reported defence ministry officials on Thursday as saying activity had been spotted at a site in Dongchang-ri, where the Sohae launching station is located.
Defence Ministry spokesman Moon Sang-gyun also said the South's military was ramping up its air defence readiness so it was ready to intercept any missile or debris falling in its territory. The South has already ordered certain commercial flights to divert their routes.
Japan's national broadcaster NHK, citing unnamed officials, also reported similar news about activity at Dongchang-ri, and added that a mobile launcher carrying a ballistic missile had also been seen moving near the east coast.
Separately, South Korean President Park Geun-hye said in a statement reported by Yonhap that any long-range missile launch by the North "should never be condoned as it poses a threat to peace on the Korean Peninsula and the world".
She said the move was "a desperate measure" by the North to maintain its regime, and showed Pyongyang was not afraid of UN sanctions.
The US-based North Korean analysis website 38 North said recent satellite images show recent activity at Sohae suggesting launch preparations.
These include heightened activity at a building used to receive rocket stages, and a complex that appears ready to conduct engine tests.
North Korean state news agency KCNA reported on Thursday that the ruling Workers' Party of Korea was holding a meeting among central and army committee members where they discussed how to "further strengthen" the party ahead of a rare political meeting scheduled for May.
Analysts say the North's recent nuclear and missile activity could be a build-up to the upcoming seventh party congress - the first to be held since 1980 - where leader Kim Jong-un is expected to show off the nuclear programme.
North Korea had told the UN agency International Maritime Organization earlier this week that it plans to launch a satellite between 8 and 25 February.
The move sparked intense warnings and criticism from South Korea, Japan, Russia and the United States.
South Korea has warned a "severe price" will be paid if the North goes ahead.
Pyongyang's main ally, China, has called on the North to "exercise restraint, act with discretion and refrain from any actions" that might escalate tensions.