Presidents Obama and Lee met on the sidelines of the G20 in London
US President Barack Obama and his South Korean counterpart have agreed a "stern, united response" must follow any rocket launch by North Korea.
Mr Obama and President Lee Myung-bak discussed the issue on the sidelines of the G20 summit in London.
North Korea says the rocket will launch a satellite between 4-8 April, but critics suspect it is a missile test.
North Korea has threatened Japan with immediate retaliation if it makes any effort to intercept the rocket.
North Korea has also warned that any attempts to impose UN sanctions would be seen as a hostile act.
'Thunderbolt of fire'
The US and South Korean leaders met before global talks on the economic crisis got under way.
"They agreed on the need for a stern, united response from the international community if North Korea launches a long-range rocket, and to work together in the course of that," the South Korean presidential office said in a statement.
There was no immediate word on the launch from the White House, but President Obama told journalists that South Korea was one of "America's closest allies and greatest friends".
Ahead of the G20, President Lee also met Japanese Prime Minister Taro Aso in London to discuss the issue.
They affirmed their intention to refer Pyongyang to the UN Security Council for sanctions if a launch is carried out - a move backed by the US.
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, and several other G20 partners are also said to back UN action.
President Lee said they should also try to convince China and Russia, security council members with veto power, to "join in a strong response".
But the International Crisis Group, a leading think tank, said China and Russia could exercise their veto on Pyongyang's behalf if a final resolution made "no mention of launches relating to peaceful outer space activities."
Japan also won backing from President Lee to shoot down the rocket if it misfires and endangers Japanese territory, said Japan's deputy cabinet secretary for public relations, Osamu Sakashita.
"Japan can rightly take any action to protect its territory," he said, and the South Korean president "completely agreed with Mr Aso's views."
The North has warned it would consider any interception "the start of Japan's war of re-invasion". Japan ruled the Korean peninsula as a colony between 1910 and 1945.
North Korea's military has threatened immediate retaliation if "even the slightest effort" is made to intercept its rocket. The military was quoted by the North's official Korean Central News Agency as specifically mentioning Japan, the United States and South Korea in this respect.
Japan was threatened with a "thunderbolt of fire" if it interfered with the launch.