A fighter jet takes off from K-55 Osan Air Base in Pyeongtaek. (Osan Air Base Facebook) Three student activists were arrested Thursday after a court approved arrest warrants over their alleged trespassing at K-55 Osan Air Base, a US air base in Pyeongtaek, earlier this week.Eight members of the Korean Progressive University Student Union attempted to enter the base through its main gate on Tuesday. Six entered the installation, while two were stopped at the entrance, according to police.US Forces Korea personnel detained the activists at the scene before handing them over to the Pyeongtaek Police Station. Four were released, while the remaining four were held in custody. Following a warrant hearing Thursday, the court approved arrest warrants for three of the four activists but rejected the request for the fourth.Police said the activists filmed themselves chanting anti-US slogans targeting the US 7th Air Force. The student group said the protest was intended to oppose what it described as US military interference in a planned semiconductor cluster in Gwangju. The student group said it was also protesting what was initially reported as a possible white phosphorus leak at Osan Air Base. USFK said Thursday that subsequent testing found no white phosphorus and determined the residue was nonhazardous surface corrosion.The case is being investigated under South Korea's Military Bases and Installations Protection Act. Earlier this week, A US military statement said unauthorized attempts to enter a US military installation are "taken extremely seriously" and would be handled in accordance with US and South Korean laws.