The father of two Cardiff jihadists thought to have been fighting alongside Reyaad Khan has said he fears his sons are on a "hit list".
Ahmed Muthana, father of Nasser, 20, and Aseel, 17, was responding to the news Khan, 21, had been killed by an RAF drone strike in Raqqa, Syria.
Prime Minister David Cameron said Khan had been plotting "specific and barbaric attacks".
He added the "act of self-defence" was lawful.
Mr Muthana said: "I think they are on a hit list. They're targeting everyone now.
"I ask the prime minister not to target them but if they have evidence I don't mind, they deserve it...but I don't think they are involved in attacks on the UK."
He added: "I think the prime minister is hiding something. He's making excuses.
"They just want to intervene in Syria.
"He's making it up when he says there's information. If he has the proof then he has the right to kill them, but he should publish that proof. I think it was done for political reasons."
Meanwhile, Muslim leaders have demanded proof Khan was plotting a terrorist attack on the UK.
A Khan family friend, Mohammed Islam, said there was a "cover-up".
The former city councillor told the BBC's Victoria Derbyshire programme: "We are very shocked and devastated to hear that a British prime minister for the first time ever in history went out of the country, we don't have any war with the country, and go and kill a British citizen without any legal permission from the parliament."
Mr Islam said Mr Cameron should have sent an "SOS" and captured Khan to "bring him to justice" in the UK.
He went on: "What is so serious that he (Khan) is going to do something from Syria and our MI6 and MI5, and the whole defence in the British system, are completely going to fail a single Reyaad Khan?
"This is completely surprising me and shocking me that the whole country and the defence system is failing [those] jihadists. This is a cover-up story, I believe."
Saleem Kidwai, Secretary General of the Muslim Council of Wales, said Khan's family knew of his death about 10 days ago but the prime minister's revelation about the drone strike was the first they had heard about how he died.
"They weren't aware how he died and that was why it was a shock to the community that it was a drone attack and he was getting involved in terrorist activities," he told BBC Radio Wales.
"The community would like to have more information on what he wanted to do for him to be killed.
"It would be better for the prime minister or government to come up with some evidence to satisfy the community."
In an earlier interview, Mr Kidwai said: "The people who knew Reyaad Khan never thought that he's the kind of a person who can do something sitting far away from here."
The prime minister told MPs Khan was killed along with Ruhul Amin, from Aberdeen, in a "precision strike" on 21 August by a remotely piloted aircraft, "after meticulous planning", while he was travelling in a vehicle.
In a Commons statement on Monday, Mr Cameron said both Khan and Amin had been involved in actively recruiting IS "sympathisers" and were plotting to attack "high-profile public commemorations" taking place in the UK this summer.
National newspapers are reporting they included VE Day in May, Armed Forces Day in June and VJ Day last month,
Two years ago MPs rejected possible UK military action in Syria, but last September approved British participation in air strikes against IS targets in Iraq only.
However, officials said the UK would "act immediately [in Syria] and explain to Parliament afterwards" if there was "a critical British national interest at stake".
The strike on Khan was "the first time in modern times that a British asset has been used to conduct a strike in a country where we're not involved in a war", the PM confirmed.
Following the announcement in the Commons, Cardiff West MP Kevin Brennan said Khan's death was a "sad inevitability".
"But the house will have been surprised to find that the manner of his death has been by a drone strike against a British citizen in Syria and therefore there will be many questions that the house will have, and I will have as his constituency MP," he added.
Mr Brennan said he wanted Mr Cameron to explain the "nature of the threat" Khan posed.
The PM responded he would be as "forthcoming as I possibly can be".
Defence Secretary Michael Fallon told BBC Breakfast on Tuesday: "This was a perfectly legal act of self-defence to prevent an armed attack on the streets of Britain."
Analysis by David Cornock, BBC Wales parliamentary correspondent
David Cameron said Reyaad Khan was behind a number of plots.
He was a terrorist plotting murder on our streets, so the prime minister said, and the only way to stop him given there was no political way to operate with the government in Syria was to kill him in the way that was done with a remote-controlled drone aircraft.
David Cameron's defence is that it was entirely lawful under the United Nations Charter that does allow self-defence by countries. He says it was signed off by the Attorney General, although he will not publish that legal approval.
Mr Cameron referred to the plots, focusing on what he called public commemorations in the UK this summer. There has been speculation [in London papers] it may have involved the VJ Day commemorations in August that were attended by the Queen
There was a real sense of shock when David Cameron outlined the details of what had happened. It is something that has not happened before, taking out a British citizen in this way in a country where Britain is not authorised in terms of conflict.
Acting Labour leader Harriet Harman asked for independent scrutiny of the decision which ultimately was approved by the prime minister himself. He will come under pressure which he will resist to publish the legal advice.
But both Harriet Harman and Kevin Brennan - Reyaad Khan's MP when he lived in Cardiff - think it right that a committee of senior MPs, an intelligence and security committee do need to look at this, because is it is as if the rules of the game have changed.