Two South Korean ministers warned activists Wednesday to stop blocking construction of a naval base on a scenic holiday island, saying the $970 million project is vital for national security.

Village residents and activists have occupied the site at Gangjeong on the coast of southern Jeju island since June, arguing the base could set off a regional arms race and damage the environment.

"We stress again the project... is absolutely vital for national security and the national interest in securing southern sea lanes," Defence Minister Kim Kwan-Jin and Transportation Minister Kwon Do-Youp said in a joint statement.

"We strongly urge the opposing groups... to stop any more protests so that we can go on with construction to create more benefits for Jeju residents and the whole nation."

The ministers said the government was running out of patience and the project could not be delayed any longer.

The volcanic island has seen its 500,000 population sharply divided since the military in 2007 chose Gangjeong as the site for the base.

Once completed in 2014 it will house some 20 warships including submarines to help protect shipping lanes through which virtually all the country's sea trade passes.

The deployment will also help guard against sea infiltration by North Korea and could quickly respond to territorial disputes with neighbouring states, the military has said.

The island's council accepted the base, citing a survey that showed a majority of islanders support it.

But opponents allege South Korea bowed to pressure from the United States, which they say is eager to establish a military presence near China.

Both Seoul and Washington, which stations 28,500 troops in South Korea, deny the accusation. Seoul's defence ministry says the new base is unrelated to US calls for a regional missile defence system.

Amid escalating clashes, the village police chief on August 24 was pelted with rice balls by angry protesters, who later prompted officers to lock themselves in a police station for protection.

Several activists were arrested last week.

More than 300 police are guarding the construction site against scores of protesters, ranging from environmentalists to religious leaders and anti-government activists.

Protest organisers said they plan to bring hundreds more activists by chartered plane Saturday to stage a mass rally, prompting police to deploy more anti-riot forces including those from the mainland.