A hijacked Singaporean oil tanker carrying 21 sailors was released on Sunday a week after being seized by pirates in waters off Ghana in west Africa, Seoul's foreign ministry said.

The 3,200-tonnes tanker, carrying a South Korean captain and chief engineer, was hijacked on July 26 off the southern coast of Ghana, the ministry said.

The vessel, which had loaded in the Pacific island nation of Kiribati, was also carrying 12 Chinese, one Singaporean and six Burmese, it said.

"All sailors are unhurt and it appears that some of their fuel has been stolen," it said, adding the ship was released in waters off Nigeria.

It did not say whether a ransom has been paid, but Yonhap news agency said the hijacking appeared to have been aimed at stealing fuel.

"There has been a sharp increase in piracy activities in waters off West Africa, but in most cases, (they) abandon the ships after taking oil, money, or valuables," said a senior Seoul official quoted by Yonhap.

"This case appears to be of a similar kind."

Piracy off the western coast of Africa has been rising in recent years, with attackers targeting ships playing a key role in the region's thriving oil industry.

The International Maritime Bureau said West African piracy made up 19 percent of attacks worldwide last year, with Nigerian pirates accounting for 31 of the region's 51 attacks -- the most since 2008.