GENEVA, April 15 (Reuters) - Almost 6,000 migrants and refugees have sailed from Libya to Italy in the past three days in what appears the start of a wave of at least 100,000 and "possibly many, many more" this year, the International Organization for Migration said on Friday.

The report may fan fears of a huge resurgence of migrant numbers reaching Europe via Italy by sea from Libya now that a shorter, safer route via Greece has been blocked, with Austria preparing to stop unhindered travel over its border with Italy in anticipation of more migrants bound for the heart of the EU.

Only a few hundred of the people who crossed the Mediterranean to Europe in the past three days entered via Greece, a massive reversal from the pattern seen until a March deal between the European Union and Turkey to stop the flow.

The voyage by boat from Libya is far longer and associated with much higher death rates than between Turkey and Greece, though Libya's lack of functioning government and lawlessness make it easy for people traffickers to operate with impunity.

"If it continues at this rate through the coming spring and summer months, we have every reason to think that arrivals to Italy will pass 100,000 for the third straight year, and could possibly be many, many more than that," IOM spokesman Joel Millman told a regular U.N. briefing in Geneva.

Two years ago about 40,000 Syrians crossed Egypt to take the Libyan route, but Millman said Syria's civil war did not seem to be driving the latest influx from North Africa.

"It could be starting again but we want to reiterate we have not seen any evidence of that. The number of Syrians that have arrived in Italy since the change in the EU-Turkey agreement could be counted on one hand, as I understand."

In 2014 some 170,000 migrants and refugees arrived in Italy, and a further 153,842 came in 2015, compared with around one million who made the relatively brief sea journey from Turkey to Greece last year.

Millman said the latest surge into Italy was unlikely to be due to smuggling gangs diverting flows of people in response to the EU-Turkey deal, since the Libyan smugglers were thought to be independent of those in Turkey.

"The Libyan smugglers that we're aware of are extremely experienced, they have been doing it a long time and they are very well armed. I think they will probably operate the way they always have, which is nationals of the migrant groups themselves mak(ing) these contacts with them."

Migrants, on the other hand, always find "niches", such as about 350 Syrians who had crossed Mauritania in the last few weeks en route to Libya's Mediterranean coastal capital Tripoli.

"Several hundred in a few weeks across Mauritania is an indicator that people are looking for openings where they can," Millman said.

Migration experts have long said that there may be hundreds of thousands of potential migrants in Libya, many from sub-Saharan Africa. "There's a huge amount of pent-up demand in Libya to get away from that situation," Millman said. (Reporting by Tom Miles; Editing by Mark Heinrich)