By Pradanna Putra Tampi and Fransiska Nangoy

NONGSA, Indonesia (Reuters) - A speedboat carrying 101 people, mostly migrant workers, struck a reef and sank off an Indonesian island on Wednesday, killing 18 people, the country's disaster agency said.

The accident occurred as the boat was heading from the southern Malaysian state of Johor toward Indonesia's Batam island, just south of Singapore, according to a statement by the Indonesian national disaster mitigation agency.

The boat, carrying 98 Indonesian migrant workers and three crew, hit the reef after being caught up in strong winds and big waves and sank at around dawn, it said.

As of Wednesday evening, 18 people were confirmed dead, 39 people had been found alive while rescue teams were searching for the remaining 44, the agency added.

A helicopter will be helping in the hunt, regional police chief Sam Budigusdian told reporters.

Zainul Arifin, one of the survivors who worked at an oil palm plantation in Malaysia, said he was sitting at the back of the boat when seawater started coming aboard.

"I had to jump off and start swimming," Arifin told Reuters in the port town of Nongsa on Batam island.

Many Indonesians work in Malaysian factories and plantations. Boat accidents are common in Indonesia, a sprawling archipelago of more than 17,000 islands.

(Additional reporting by Yuddy Cahya and Heru Asprihanto; writing by Eveline Danubrata; editing by Mark Heinrich)