Four South Korean sailors have been set free more than 19 months after they were seized by Somali pirates, after paying an unspecified amount of ransom, a Seoul official said Saturday.

The four South Koreans were kidnapped by Somali pirates on April 30, 2011 aboard the MT Gemini, a tanker operated by Singapore-based Glory Ship Management, along with 21 non-Korean sailors. The ship was en route from Kenya to Malaysia at that time.

Seven months later, the pirates freed the vessel, but broke an agreement to release all of the crew, keeping the four Koreans captive while releasing the other 21 non-Korean crew members.

"Negotiations between the Singapore-based firm and Somali pirates to free the Korean sailors have ended successfully," the South Korean foreign ministry official said, speaking on the condition of anonymity.

The four South Koreans -- the MT Gemini captain Park Hyun-yeol, chief engineer Kim Hyeong-eon, chief mate Lee Geon-il and engineer Lee Sang-hoon -- were released as of 5:55 p.m. (Seoul time), the official said.

"Our sailors are now under armed guard on a destroyer of the Cheonghae Unit," the official said, referring to the South Korean naval contingent that has been deployed in the Gulf of Aden.

"The four men will return home as soon as possible after undergoing medical checkups at a hospital in the region," the official said, adding that the crew were not in bad health.

The release came after ransom talks between the Singapore-based owner of the MT Gemini and the Somali captors made progress after the pirates withdrew their demand that five of their colleagues serving terms in South Korean jail be set free. The five were arrested during a South Korean commando raid in 2011 on the South Korean tanker MV Samho Jewelry that was seized by Somali pirates in the Gulf of Aden.

The official said the Seoul government had assisted in ransom talks between the ship owner and the pirates, but declined to say how much ransom had been paid.

After two decades of civil war, Somalia is taking steps to have a functioning government.

South Korea has been deploying a destroyer with some 300 special Navy troops since early 2009 in the Gulf of Aden as part of the U.S.-led multinational anti-piracy campaign.

Another ministry official said the South Korean sailors would be flown to Seoul Wednesday or Thursday.

"As negotiations between the Singapore-based firm and the pirates had made big progress, the government swiftly dispatched this week its team and the Cheonghae unit to the area," said the official who also spoke on the condition of anonymity.

The families of the four released sailors were jubilant at the news.

"It is a huge relief to know that my brother is safe and would return home. I couldn't hold back my tears," said Park Hyun-ae, a younger sister of the 57-year-old captain, Park Hyun-yeol.

She, who lives in the southern port city of Busan, said she had received a telephone call from her brother about 40 minutes after the news broke out.

"His first word was, 'It's your brother, I'm coming home now,'" the sister told Yonhap News Agency.  (Yonhap News)