The state-run Korea Aerospace Research Institute said Yi is undergoing MRI and other scans at an air force hospital to determine the exact cause of her discomfort.

"She has complained of considerable back pains and will have to cancel all her appointments for the time being, including visits to the presidential office and TV interviews," a doctor at the military hospital was quoted by Yonhap news agency as saying.

Yi and two colleagues returned to Earth in what some Russian media called a dangerous re-entry on April 19, when her Russian-designed Soyuz capsule landed hundreds of kilometres off target.

Interfax news agency said the capsule was facing the wrong direction when it entered the atmosphere, depriving it of the protection of its heat-resistant shield.

The landing subjected the crew to huge gravitational forces, and experts quoted by Yonhap said that Yi may have received more shock than the other two members as the capsule hit the ground near her seat.

Yi told an airport press conference on her return home Monday that she has some lingering pain.

The 29-year-old biosystems engineer carried out 18 experiments, including biological, geophysical and medical tests, during her nine-day mission at the International Space Station orbiting the Earth.

South Korea paid some 20 million dollars for her mission, becoming the 36th country to send a astronaut into space.

Seoul is scheduled to launch a satellite later this year from its own space centre, under construction at the country's southern tip. It plans to launch a lunar orbiter by 2020 and send a probe to the moon five years after that.

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