"It is tragic that we lost two of the tugboat crew, while two others are still missing," Prime Minister Pravind Jugnauth said after visiting rescued sailors in hospital.
Jugnauth said the tug was transporting some fuel at the time "but there is no risk of a leak". The prime minister promised an investigation into the accident.
He faces growing anger over his administration's handling of the oil spill, which has caused untold ecological damage to a protected coastline that sustains the island's economy.
A search was under way for the missing crew after the accident late Monday as the boats returned from where a tanker crashed into a reef in late July, leaking more than 1,000 tonnes of oil into the island's picturesque waters.