SEOUL, South Korea (AP) — South Korean police said Saturday they beefed up security at the U.S. ambassador's residence in Seoul after a group of anti-American students used ladders to break into the compound.

They were protesting demands by the Trump administration that South Korea pay more to help cover the costs of keeping U.S. troops in the country.

Officials from three Seoul police stations didn't immediately say whether they will seek to formally arrest any of the 19 university students who were detained Friday afternoon at the residence of Ambassador Harry Harris.

The Seoul Metropolitan Police agency said Saturday the number of officers guarding the residence was more than tripled to 110.

The demonstrators, who broadcast parts of their protest on Facebook, used two steel ladders to climb over the compound's wall. They shouted anti-U.S. slogans and held up signs that read "The United States has called for a five-fold increase in defense costs!" and "Harris, leave this land!" before police officers dragged them out.

The U.S. State Department has expressed "strong concern" over the illegal entry and urged South Korea to strengthen its efforts to protect all diplomatic missions.

South Korea's Foreign Ministry criticized the protest, saying in a statement that "any attack or harm inflicted on foreign diplomatic missions cannot be justified under any circumstances." It said the government will take "every appropriate measure" to protect the facilities.