North Korea is believed to have placed a new, powerful mid-range missile on standby for an impending launch, reports claim.
The reported launch plans came two days after North Korea claimed to have successfully test-fired a ballistic missile from a submarine in a continuation of its weapons tests amid ongoing South Korea-US military drills.
Yonhap news agency cited an unidentified Seoul official as saying South Korea's military has unspecified evidence indicating North Korea will likely soon launch a mid-range Musudan missile.
Seoul's Defence Ministry says it has no such intelligence.
Yonhap says the missile on standby is one of the two Musudan missiles North Korea had earlier deployed in the north-east before it fired one earlier this month.
US officials say the earlier launch ended in failure.
A Musudan has a reach of 2,180 miles, putting far-off US military instalments in Asia in range.
North Korea typically conducts more weapons tests when South Korean and US troops conduct springtime drills that it views as an invasion rehearsal.
This year's drills end later this week.
Last week, South Korea's president said there were signs North Korea was preparing for a fifth nuclear bomb test amid media reports of increased activity at the country's main nuclear test site.
North Korea carried out a fourth nuclear test in January.
Analysts say a new atomic test could happen before North Korea holds a ruling Workers' Party congress in early May so that leader Kim Jong Un can burnish his image as a powerful leader at home and further cement his grip on power.