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The US has formally asked Australia to expand its role fighting against Islamic State (IS) to include Syria, local media reported.

The request was reportedly sent to the Australian embassy in Washington on Thursday.

The government would say only that the request would be "properly considered".

The move would allow the Australian air force, already involved in air strikes over Iraq, to pursue IS targets in Syria as well.

Federal Treasurer Joe Hockey, who sits on Australia's national security committee, said IS did not recognise borders and Australia had an obligation to "do what we can" to stop them.

The Australian newspaper reported that the US request included enhanced intelligence gathering, support for US and coalition fighter planes in Syrian air space and the potential for Australian airstrikes.

'Not a game changer'

The Australian air force has been bombing IS targets in Iraq for about 12 months.

Government backbench MP and chairman of Federal Parliament's intelligence and security committee, Dan Tehan, has called on the government to broaden its commitment to fighting IS.

Earlier this week, Australia's chief of Joint Operations Command, Vice-Admiral David Johnston, said the air force operated along the Syrian border but did not cross it.

He played down any greater role for Australia, saying it would not be a "game-changer".