If the missile launch happens it will be a direct violation of the UN Security Council resolution, exacerbating the security scenario in the region following Pyongyang's 'successful' testing of its first hydrogen bomb earlier this month.
CNN has reported that images from US spy satellites, which keep a sharp eye on North Korea's secretive Sohae Satellite Launching Station at Tongchang-ri, have indicated increased activity at the site.
While North Korea has said the site is use for launching satellites into space, the US and Japanese intelligence officials assume the North's plan has more to do with military strategy.
North Korea has been attempting to build an intercontinental ballistic missile (ICBM) and its last rocket launch was in 2012, when it said it put into orbit a communications satellite.
South Korean Defence Ministry declined to comment on speculation about pre-launch activities at the north's launch site, Reuters reported.
However, the ministry spokesman said in a regular press conference the country's military is "prepared for various types of North Korea's provocation and is ... closely and continuously observing any signs of North Korea's long-range missile launch."
Meanwhile, the Yonhap news agency said there has been steady activity at the missile base, citing government official.
The agency reported site was upgraded last year to accommodate the launch of a longer-range rocket, citing experts.
US Secretary of state John Kerry was in Beijing on Wednesday where he asked the Chinese authorities to rein in North Korea's nuclear programme and step up actions against the reclusive state whose only serious ally is China.