Japan on Tuesday approved a plan to send a unit of ground troops to South Sudan as part of a UN nation-building force, where they are expected to help construct infrastructure for the fledgling nation.

Japan's military, called the Self-Defence Forces, is barred from fighting overseas under the country's pacifist post-World War II constitution, but it has joined UN peacekeeping forces in countries such as East Timor and Haiti.

Under the latest plan, troops will be deployed to the South Sudanese capital of Juba -- which is considered relatively safe -- but will be permitted to use weapons in self-defence.

The troops, mostly engineers and logistical staff, are expected to help repair or build roads, bridges and infrastructure in the landlocked African country, which declared independence from Sudan in July after a long civil war.

Defence Minister Yasuo Ichikawa ordered the Self-Defence Forces to prepare for deployment and await further instructions on their mission, a defence ministry official said.

"I've always thought that the engineering unit of the Self-Defence Forces should play a role, so that they can leave footprints in South Sudan's nation-building," Ichikawa told reporters.

"As we start the new mission, it is important for Japan to show the world that we are playing a role in the international community," he said.

Japan, which has already dispatched two fact-finding teams to South Sudan, is looking at sending the first batch of about 200 troops early next year to establish bases, Deputy Chief Cabinet Secretary Tsuyoshi Saito said.

The main unit of around 300 troops will replace them later, he said.

South Sudan, one of the world's poorest countries, became a member of the United Nations on July 14 and joined the African Union on July 28.

Japan has contributed military forces to other non-combat operations, including the reconstruction mission in Iraq and as part of anti-piracy patrols off Somalia.

The Iraq mission, however, was deeply unpopular with the Japanese public, as opponents said joining the US-led coalition violated the pacifist constitution adopted in the wake of defeat in World War II.