Heavily armed gunmen fired on an oil tanker off the coast of Nigeria in the latest in a spate of pirate attacks in the increasingly treacherous Gulf of Guinea, a maritime watchdog said Thursday.The Panamanian-flagged, Nigerian-owned vessel thwarted an attempted boarding by taking evasive action during the attack, which occurred Wednesday at about 2300 local time (2200 GMT), the International Maritime Bureau (IMB) said.It came one day after pirates shot their way onto a Dutch-owned cargo vessel in the area and kidnapped three hostages including the captain, according to the IMB’s Kuala Lumpur-based piracy reporting centre.Noel Choong, who heads the centre, said no further details were available on the most recent incident, warning that the attackers may still be in the area.The assault occurred about 80 nautical miles (148 kilometres) south of the Nigerian coast, the centre said.Agency