North Korea's missile move comes amid tense ties with Seoul
South Korea, Japan and the US are garnering support on the sidelines of the G20 summit in London for UN sanctions against North Korea.
The North has said it plans to launch a satellite between 4-8 April, which other states believe will be a missile.
It has said any UN action would be seen as a hostile act and any attempt to intercept its launch an act of war.
Reports based on an unnamed US military source suggest the North is fuelling a rocket to ready it for launching.
A Japanese official said in London that it had secured the backing of several of its G20 partners including South Korea and Britain to refer North Korea to the UN Security Council.
"We should try to convince China and Russia to join in a strong response" such as taking the matter to the Security Council, South Korean President Lee Myung-bak told Japanese Prime Minister Taro Aso.
In personal meetings in London on the eve of the G20 summit, Mr Lee stressed the need for a "united response" among world leaders to the threat from the North.
British Prime Minister Gordon Brown and Australian Prime Minister Kevin Rudd have already expressed their support.
"The launch will clearly constitute a violation of the Security Council resolutions, so it needs to be discussed in the appropriate manner in the council," said Osamu Sakashita, Japan's deputy cabinet secretary for public relations.
He said that Mr Lee told Mr Aso that Japan was entitled to shoot down the rocket if it misfires and endangers Japanese territory.
"Japan can rightly take any action to protect its territory," the official said, and the South Korean president "completely agreed with Mr Aso's views."
He said Japan had also been cooperating with the United States "to be prepared for any eventuality."
The North has also warned it would consider any interception "the start of Japan's war of re-invasion."
North Korea's military has threatened immediate retaliation if "even the slightest effort" is made to intercept its rocket.
The North's official Korean Central News Agency quoted the military as specifically mentioning Japan, the United States and South Korea, threatening Japan with a "thunderbolt of fire" if it interfered with the launch.
US President Barack Obama is expected to join the push for a joint response when he meets with the South Korean leader on Thursday.
China and Russia have veto power in the security council and could exercise it on North Korea's behalf, a leading think tank has warned.
The International Crisis Group said the two states could cite legal uncertainty over the wording of the resolution because it makes "no mention of launches relating to peaceful outer space activities," the Brussels-based group said in a report.
The US has argued that no matter what is launched on the rocket, the likely proof of the North's rocket-launching capability alongside its arsenal of dangerous weapons remains a problem.
A US official, speaking on condition of anonymity in London said Washington would respond to any North Korean launch by raising it in the UN.
"The president made clear we are deeply concerned about the prospective missile launch by the North Koreans... There will be a reaction to it," the official said.
CNN reported Wednesday, citing an unnamed US military official, that North Korea has started fuelling a rocket ready for launch.
South Korea's Defence Ministry said it was aware of the report but declined to comment.