South Korea denied entry on Monday to three Greenpeace activists and was to deport them after they attempted to get into the country as part of an anti-nuclear campaign.

Mario Damato, one of the detained trio, said his treatment highlighted South Korea's "growing willingness to suppress voices speaking out against its nuclear energy expansion ambitions".

"Our deportation is a wake-up call for the people of South Korea of what they can expect if their country expands its already unhealthy reliance on nuclear power and allows this kind of crackdown to continue," Damato said in a Greenpeace statement.

"It is absolutely unacceptable for the authorities to shrink the democratic space and pressure legitimate voices of concern. We will resist any attempt to silence us," he added.

South Korea relies on 20 nuclear reactors to meet about 35 percent of its electricity needs. The government has vowed to stick to the nuclear pogramme despite heightened concern following Japan's atomic disaster last year.

"Their entry was denied at the request of security authorities. They are to be deported soon," an immigration official told AFP, declining to give details.