Pirates tend to target cargo ships such as the MV Savina Caylyn, which was hijacked earlier this month [AFP]

A US-flagged vessel, with four American citizens on board, is reported to have been hijacked off the coast of Oman, an official at the US embassy in Nairobi, the Kenyan capital, has said.

"Preliminary reports indicate that a US-flagged vessel tentatively named as the Quest has been hijacked in the Arabian
Sea. There were four US citizens on board," the spokesman said.

Earlier on Saturday, Andrew Mwangura, a regional maritime expert, said tht the 58-foot S/V Quest had been hijacked 240 miles off Oman, as it sailed from India to Salalah in Oman.

Ecoterra, an advocacy group monitoring piracy, said in a statement that the yacht was owned by Jean and Scott Adam.

Pirates operating in the Indian Ocean have hijacked dozens of vessels, ranging from yachts to large oil tankers, in recent years, making millions of dollars in ransoms.