An Indian warship has warded off another attack by pirates on merchant vessels carrying 40 Indians among its crew south of Yemen, the fourth in the last 10 days, the Navy said here on Thursday. INS Delhi, a guided missile destroyer, was escorting six merchant vessels with 147 crew members, including 40 Indians on board, on Wednesday when a dhow was spotted approaching the formation at a high speed. The formation escorted by INS Delhi was sailing along the International Recommended Transit Corridor, 108 km away from the south Yemeni coast, when the sea brigands made the attempt on the merchant vessels around 1000 hours. The warship launched its Marine commandos, who intercepted and boarded the dhow. The commandos found a cache of arms including a 7.62 mm rifle, six rocket propelled grenades and ammunition on the dhow. "This is the fourth such action by INS Delhi, three of which was taken in the last four days," a navy spokesperson said here. INS Delhi had warded off similar attack on September 5, 12 and 13 earlier this month. Since the Navy started its operations against pirates in Gulf of Aden in October 2008, this is the 18th attack that was prevented by the Indian warships.