Indonesian authorities have rescued a tug boat with nine crew believed to have been hijacked in the Straits of Singapore, a sea lane seeing a rise in attacks, an official said Monday.

The boat was recovered on Saturday off Indonesia, said Noel Choong, head of the piracy reporting centre of the International Maritime Bureau (IMB).

It had been towing a barge laden with heavy machinery and piling materials which an Indonesian aircraft and three warships found in the same area a day later, he added.

The tug and barge, heading from peninsular Malaysia to Borneo island, last made contact on December 27 off southern Malaysia near the Straits of Singapore before it was hijacked, Choong said.

"IMB is very pleased with the Indonesian authorities' quick response because of which we managed to recover the tug and barge," he told AFP.

Choong urged authorities to step up patrols and ships to be vigilant in the Straits of Singapore, which saw 11 attacks last year, up from three in 2010.

Malaysian maritime authorities in October rescued a tanker they said was also hijacked in the straits, which runs between Singapore and Indonesia.

In September, a Malaysian court sentenced six Indonesians to 10 years in jail and caning for trying to rob a merchant ship near the same area.

The nearby Strait of Malacca -- running between Indonesia and Malaysia and once the most hazardous shipping channel -- has seen a lull in attacks in recent years following cooperation by the surrounding countries to rein in piracy.

Singapore, Malaysia and Indonesia increased patrols two years ago amid concerns that pirates planned to attack oil tankers.