A police official said investigators believed three Yemeni men found on the trawler were hostages of the alleged pirates, who are believed to have hijacked the vessel two weeks ago off the coast of Somalia.

The Yemenis told police the boat had been adrift in the Indian Ocean after running out of fuel. All 17 men were detained in the coastal district of Junagarh in Gujarat state, 300km southwest of the main regional city of Ahmedabad.

"After interrogation we ascertained that the Somali nationals are pirates who had kidnapped the three Yemenis," Junagarh police chief Depankar Trivedi told AFP by telephone.

He said police would press charges against the Somalis and their captives for entering India without valid travel documents.

The official said the Somalis were only charged with trespassing because the hijacking occurred beyond India's jurisdiction in international waters. "We are charging them only for the violation of Indian laws," he said. The Yemenis did not possess travel documents.

Trivedi said a marine police team detained the men on Sunday after local fishermen reported the presence of the Yemen-flagged vessel only a couple of kilometres off the coast of Gujarat.

India's coastguard and navy are on high alert against pirates seeking to evade the international force patrolling waters off Somalia by attacking shipping much further east in the Indian Ocean.

More than 100 pirates have been caught and are awaiting trial in India following a series of violent skirmishes near the country's Lakshadweep islands since the start of this year. India is still planning to frame legislation to deal with the scourge of piracy.