By Matt Siegel

SYDNEY (Reuters) - Australia will accept 12,000 refugees from Syria on top of its current humanitarian intake quota and extend airstrikes against Islamic State group militants in Iraq into Syria, Prime Minister Tony Abbott said on Wednesday.

Abbott on Sunday said Australia would allocate more spaces in its 13,750 annual quota to those fleeing violence in Syria, but did not plan to boost the overall arrivals, sparking criticism from across the political spectrum.

The one-off move to offer refuge to those fleeing the four-year-old civil war in Syria would prioritize members of persecuted minorities in that country, Abbott said.

"Australia remains committed to the international effort to counter Daesh, which threatens stability in Iraq and theMiddle East and the security of Australians at home and in our region," he told reporters in Canberra, using an Arabic name for Islamic State.

At least 850,000 people are expected to cross the Mediterranean seeking refuge in Europe this year and next, theUnited Nations said on Tuesday, giving estimates that already look conservative. The U.N. refugee agency UNHCR called for more cohesive asylum policies to deal with the growing numbers.

The Royal Australian Air Force is already taking part in the U.S.-led coalition campaign against Islamic State targets in Iraq, but its aerial role in Syria has so far been limited to refueling and intelligence gathering.

The decision to expand air strikes was in response to a formal request from Washington, Abbott said.

"This is not an attempt to build a liberal pluralist market democracy overnight in the Middle East. That's been tried and it didn't magnificently succeed," Abbott said.

"Surely all human beings are entitled to a government which doesn't commit genocide against them."

Australia also committed to directly pay for the support of 240,000 displaced people in countries neighboringSyria and Iraq, Abbott said, at a cost of A$44 million ($31.03 million).

Australia's tough stance on asylum seekers, which Abbott argues is necessary to stop deaths at sea, has been strongly criticized by the United Nations and civil society groups.

(Reporting by Matt Siegel; Editing by Nick Macfie)