The Abbott government will spend at least a week considering whether Australia should join the fight against Islamic State in Syria.



But the prime minister, Tony Abbott, has said that while the legalities of helping with air strikes over Syria were different to Iraq, where the government invited Australia to fight, the moralities were exactly the same.



“The terrorists don’t respect the border, so why should we?” he said to reporters in the Kimberley region of Western Australia on Sunday.



The foreign affairs minister, Julie Bishop, said that while the US had formally asked Australia to expand its operations in the Middle East to include air strikes on Syria, the government wanted to be sure there was a credible legal basis for such an action.



She indicated a final decision may not be made until late September, after Abbott meets with the US president, Barack Obama and the crucial federal byelection in the WA seat of Canning.



However, Abbott said his government would be carefully considering the US request “in the next week or so” and announce its decision after appropriate consultations.



Bishop said the situation was complicated by the non-recognition of the Assad regime in Syria but the US believes that because the border region is effectively “ungoverned space”, it has the legal authority necessary to bomb Islamic State extremists there.



The US has asked Australia to support air strikes and carry out intelligence surveillance, reconnaissance and air refuelling in Syria’s border areas.



While there has been bipartisanship on tackling terrorism, some cracks are beginning to appear.



Bishop took aim at Labor’s deputy leader, Tanya Plibersek, on the issue on Sunday. She said the government had opposition leader Bill Shorten’s full support but his deputy always seemed to take another stance.



“She says that she supports the Labor position, the Coalition position, but then she always moves a little bit to the left and has a slap at Bill Shorten on the way through,” Bishop told ABC television.



Opposition frontbencher Brendan O’Connor said Labor had sought to work with the government on national security at all times.

“We would like to get a full briefing before making our decision [on Syria],” O’Connor told Sky News.