Two of the men were on a vessel known to have sunk when pounded by 190 kilometre-an-hour (119 miles per hour) winds and 15-metre (50-foot) seas, and the American owner of another boat with seven people on board said he had not heard from his crew since Wednesday.

New Zealand Rescue Centre spokeswoman Heidi Brook said fears were held for the safety of the missing men, as a New Zealand Air Force Orion, a US Coastguard Hercules and local vessels searched a vast stretch of ocean.

Vessels further south were also warned of looming very rough seas as Olaf headed towards the Cook Islands, although forecasters said that on its present track it would miss inhabited areas.

In Samoan waters, the Orion was focussing on locating the two men whose fishing boat was known to have sunk on Wednesday about 130 nautical miles to the north. They were among a crew of six, of whom four were found floating in the water 24 hours later.

The Hercules was concentrating on find the second vessel, also with a crew of six who were caught in the path of the cyclone about 240 kilometres away.

Fourteen distress beacons were monitored when Olaf pounded the area.

Overnight, a commercial vessel reached an American-registered fishing boat east of American Samoa, and after repairs it was heading back to Samoa, while another fishing boat with three injured crew was under tow.

There were no reports of death or injury on land from Olaf, but on American Samoa's Manua'a islands, which suffered a direct hit, Olaf destroyed many homes close to the sea, downed crops and littered the island with debris.

Ale Filoialii on the Manua'a island of Tau said nearly all homes still standing had lost rooftops and the island could be without electricity for at least three weeks.

In the Cook Islands, were residents are still clearing away the damage and debris of two other cyclones in the past 12 days, the forecast was encouraging as Olaf began to veer away from populated islands.

It was expected to pass through the area late Friday or early Saturday, although it had weakened slightly to a Category 4 cyclone, one step below the maximum Category 5.

On its present course "minimal damage would result to either Niue or the Southern Cooks although seas would be very rough and high wind warnings or even gale warnings would be in effect," the Australian-Pacific Centre for Emergency and Disaster Information said.

However, it warned that a modest deviation to the east would increase the threat to the southern Cooks or a deviation westward would heighten the risk for Niue.

"They're certainly not looking forward to Cyclone Olaf, I can tell you that," the New Zealand High Commissioner to the Cook Islands, Kurt Meyer, said from the main island of Rarotonga.

"The whole of the island spent yesterday in a clean-up mode, trying to clear the roads, clear the power lines that had come down, trees that had fallen all over the place," he said.

Meyer said the Cook Islands was able to deal with the clean up at this stage but was aware New Zealand French and Australian assistance was available if required.

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