Britain is drawing up contingency plans for military action in response to the chemical weapons attack in Syria.
Downing Street stressed any action would be "proportionate", lawful and follow agreement with international allies, the BBC reports.
Prime Minister David Cameron will consider a recall of Parliament when he returns to London later, amid growing pressure from MPs.
The chemical attack took place last Wednesday near Damascus, and reportedly killed more than 300 people.
Syrian rebels say the Assad government was responsible but it blames rebel forces, saying footage of the attack was "fabricated".
A government spokesperson says Britain is considering a "proportionate response" to the attack after United Nations weapons inspectors visited five sites around Damascus on Monday to try to ascertain what happened.
They gathered what the United Nations called "valuable" evidence from one site.
The BBC says Britain believes any use of chemical weapons is "completely unacceptable" and "abhorrent".
Obama and Rudd talk on Syria
Australian Prime Minister Kevin Rudd says the world cannot turn a blind eye to the use of chemical weapons in Syria.
Mr Rudd and US President Barrack Obama have spoken to each other about the crisis in Syria and possible responses.
During an election-campaign speech on Tuesday, Mr Rudd was asked whether he was comfortable supporting the opposition forces in Syria when they are not inclusive or secular.
Mr Rudd said there were no perfect choices. "Whatever is the domestic politics of Syria right now, and it is complex beyond description, the external principle is alive and well, which is this.
"I do not believe the world can simply turn a blind eye to the use of chemical weapons against a civilian population resulting in nearly 300 deaths or more and some 3,600 people hospitalised."
Mr Key says he has not been briefed on that conversation but New Zealand will make its own decision, if and when it becomes necessary.
Dr Peter Dean from the Strategic and Defence Studies Centre at the Australian National University told Checkpoint Mr Obama is pulling together support and Mr Rudd is positioning himself.