No casualties were reported from the attack, they said, noting the captain and a navigator sustained bruises during their confrontations with the pirates.

Besides the cash, some sailors lost their personal belongings, including mobile phones, clothes and shoes, to the attackers, they added.

The CK Bluebell en route to Incheon after departing from Brazil resumed its normal navigation after the attack, they said.

"Ordinary freighters navigate at a speed below 15 knots. But a speed boat operated by the pirates approached the freighter at a speed above 20 knots. Afterward, seven pirates, including one carrying a gun and two armed with knives, got on board and assaulted the sailors for about 30 minutes," a ministry official said.

The official said the ministry has delivered news of the pirate attack to all Korean-registered vessels near the Singapore Strait and asked them to stay alert.

There were four Koreans and 18 Indonesians aboard the CK Bluebell, which was transporting 68,000 tons of Brazilian corn bound for South Korea after a refueling stop in Singapore. The ship didn't have any special Marine guard carrying weapons. It was scheduled to arrive in Incheon on July 30.

The Korea Coast Guard, which was notified by its Singaporean counterpart of the incident at 6:33 a.m., said it has asked the CK Bluebell to secure the fingerprints of the pirates and plans to conduct a further investigation following the ship's arrival in Incheon. (Yonhap)