They staged a surprise protest criticizing the US demand for a hefty rise in South Korea's share of the cost for the upkeep of the 28,500-strong US troops.

There have been reports that the US wants South Korea to pay $5 billion under next year's cost-sharing deal, up from 1.04 trillion won ($871 million) this year.

The protest came five days before Seoul and Washington begin the second round of defense cost-sharing talks in Hawaii.

Police said they released 10 other students by Saturday, though the students will remain under investigation.

On Friday, police took into custody two others who attempted to break into the residence.

The US Embassy urged South Korea to strengthen its efforts to protect all diplomatic missions as it noted "with strong concern"

that the break-in was the second incidence of illegal entry into the ambassador's residential compound in 13 months.

South Korean police have deployed about 80 additional personnel, bringing to about 110 the total number of personnel protecting the residence.

"Big shout out to Embassy guards & Seoul Metro Police Agency for responding to protesters who breached perimeter around my residence," Harris tweeted.

In South Korea, police are required to ask prosecutors to seek a court-issued warrant to formally arrest a suspect. (Yonhap)