Labour's former acting leader Harriet Harman has questioned whether the "extra-judicial killing" of a Cardiff jihadist can be properly justified.
Reyaad Khan, 21, died in an RAF drone strike targeting Islamic State militants in Syria last August.
David Cameron told MPs on Tuesday the attack was "necessary and proportionate" to protect the UK.
Ms Harman said such action outside a war zone was a "new departure" and needed to be "properly accountable".
In September, the prime minister announced that Khan and Ruhul Amin, from Aberdeen, had been killed by a UK drone strike in Syria.
MPs have since voted to authorise UK air strikes in the war-torn country.
Questioned by MPs on the Commons Liaison committee on Tuesday, Mr Cameron defended the drone attack which killed Khan and Amin as "necessary and proportionate".
He said the UK was currently engaged in an operation against a terrorist group that intends to "blow up, kill and maim our citizens".
Ms Harman, a member of the committee, told BBC Radio Wales on Wednesday she was concerned at the lack of oversight of the operation.
"If you're a member of the armed forces, you kill somebody in a military situation, that is legal and defensible and because you're in an armed conflict situation," she said.
"But where we were in Syria, where Reyaad Khan was targeted and killed, we were not actually in a war conflict situation there.
"It was just a targeted, what's called extra-judicial killing."
She added: "The idea of killing somebody in an area where the security services have targeted them and military carries out the killing, that is a new departure.
"The Americans have done it for quite a long time but we've not done it before.
"If the police kill somebody in the line of duty, here in this country, it's automatically referred to the independent police and crime commissioner, not because there's an automatic assumption they've done something wrong.
"With the taking of a life you must make absolutely sure that it was the right person, that the evidence was sufficient, that they did it in such a way that it didn't kill innocent bystanders and that has to be looked at afterwards."