But John Berry says the final decision is up to the Australian government.
The national security committee of cabinet is expected to next week consider whether RAAF Hornet bombers, currently only operating in Iraq, would be able to cross the border into Syria.
The move came after US President Barack Obama personally raised the prospect of Australia's role in Syria with Prime Minister Tony Abbott last month.
Ambassador Berry said Islamic State did not respect the Iraq-Syria border, and military coalition partners needed greater flexibility.
"It's a combined effort that allows us to target and to track and to follow ... so that no one (in IS) can seek a safe zone," Mr Berry told reporters in Canberra on Wednesday.
Mr Berry said Australia's effort, which also includes training Iraqi forces in Taji, was making a "huge difference" in rolling back IS, or Daesh.
"We have asked for a little (more) assistance," he said. "We ought not let (IS) ... sneak across and feel that they can be untouched."
Mr Abbott confirmed on Wednesday the request had come from Mr Obama during a discussion about other issues, including the Trans-Pacific Partnership trade agreement.
"I said that I would have our officials talk with each other, and as a result of those conversations a formal request has now come from the Pentagon," the prime minister said.
Australian Strategic Policy Institute executive director Peter Jennings said the legal basis would depend on the scale of the extra effort.
If it was a case of only pursuing IS targets close to the Iraq-Syria border, then the idea of "collective self-defence" in relation to the Iraq operation would apply.
If the new air strikes were aimed at hitting IS strongholds in Syria then the doctrine of "responsibility to protect" would apply.
"The US, Canada and a number of Arab League countries have been conducting strikes in Syria for close on a year," Mr Jennings told AAP.
"It would be news to me if critics of the proposed Australian operations are now asserting these strikes are not legal."
Former Labor foreign minister Bob Carr says Australia has a moral obligation to accept the US invitation, where there is a chance of saving civilians from mass atrocities. Opposition Leader Bill Shorten has received a briefing and is seeking further information, saying Labor wants to ensure there is a legal and moral case for bombing Syria.
Independent MP Andrew Wilkie said going into Syria would be "one of the most reckless things this government has ever done".
Defence Minister Kevin Andrews insists Australia will play no part in the Syrian civil war, and any air strikes would be directed solely at IS terrorists and only in Syria's east.
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