Four South Korean nationals have been released after being held as hostages by an unidentified gang for four days in the Philippines, Seoul's foreign ministry said Tuesday.

The release was made late Monday, four days after a 35-year-old man, only identified by his surname Cho, and another three men were kidnapped in the city of San Juan on Thursday, according to officials.

They were found on a road outside Manila after being beaten by the suspects, but they did not sustain any serious injuries, they added.

"Police in the Philippines are trying to round up the kidnappers as the safety of the abductees was confirmed," a foreign ministry official said.

The suspects were known to have demanded a ransom of some 200 million won (US$185,065) in exchange for their release.

Part of the ransom was delivered to the kidnappers during negotiations to free the hostages, the ministry said, without saying how much.

Violent crimes such as murder and robbery targeting South Koreans have been on the rise in recent years in the Philippines.

Ten South Koreans were killed there last year while 13 Korean nationals were murdered there in 2013, according to the foreign ministry. In 2014, three cases of abduction of South Koreans took place.

South Korea imposed a special travel warning on Mindanao, the Philippines' second-largest and easternmost island, on Sunday as crime targeting South Koreans there has increased. The travel warning calls for immediate withdrawal from the region. (Yonhap)