Defence secretary argued that every Isis member places themselves at risk as he refused to confirm reports of Jones’s death

British nationals fighting with Islamic State in Syria are a “legitimate target” for drone strikes, the defence secretary has said, as he refused to confirm whether he believed reports that Sally Jones, who travelled to Syria in 2013, had died in a US strike.

Isis fighters were at risk of being hit “every hour of every day”, Michael Fallon said at a Foreign Office press conference.

The Sun reported on Wednesday that the CIA had told its UK counterparts Jones was killed by a Predator drone strike near the Syria-Iraq border in June. It said news of her death was not made public amid fears that her 12-year-old son, Jojo, may have been killed alongside her. US military spokesmen and US sources also refused to confirm whether Jones had been killed, or whether her son was with her at the time.

Fallon said: “If you are a British national in Iraq or Syria and if you have chosen to fight for [Isis] – an illegal organisation that is preparing and inspiring terror attacks on our streets – then you have made yourself a legitimate target and you run the risk every hour of every day of being on the wrong end of an RAF or a United States missile.”

Although deaths by drone strikes in Syria are very hard to confirm categorically, given the inability to collect evidence on the ground, there is confidence that Jones has died. It it is understood that the strike was not based on intelligence requiring the specific sanction of the UK prime minister, Theresa May.

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David Cameron, May’s predecessor, did give sanction for strikes against UK foreign fighters in Syria.

Jones, dubbed “the white widow” by some in the press, was a regular propagandist on social media. She had more than 20 accounts on Twitter, but has not been active for several months.

The UK and US are working closely together in Syria, specifically in the campaign to free Raqqa from Isis. The city in north-west Syria is now the home to fewer than 400 jihadis and possibly as few as 5,000 citizens.

Jones was believed to have been in Raqqa, but seeking to flee over the Iraq border. Raqqa has been subject to a steady coalition bombardment, and the chances of her survival in the town were always going to be minimal.

Although the US forward base for its operations in Syria is based in Qatar, the drone strikes are operated from the US and launched on the basis of human and signal intelligence gathered inside Syria. Some of that intelligence comes from the Syrian Democratic Forces, the US-backed forces operating with US air support to free Raqqa from Isis control.

Facebook Twitter Pinterest No activity on Twitter in recent months … Sally Jones. Photograph: Stewart News/Rex

British drone pilots operate Reaper and Predator drones out of RAF Waddington in Lincolnshire and Creech US air force base in Nevada.

By law, Jones’s son is too young to be deemed a legitimate target, and British sources have not confirmed that they know he is dead. If Sally Jones was killed while in a vehicle, it would have been hard to identify whether she was accompanied by her son. If, however, she was seeking to escape over the Iraq border, it is unlikely that she would have left her son behind.

Ryan Dillon, spokesman for Operation Inherent Resolve, the Baghdad-based coalition of anti-Isis forces, refused to confirm her death, but said coalition attacks on Isis do not discriminate by nationality.

The UK government is aware that there is an intense legal debate about the targeting of British recruits fighting for Isis in Syria.

In total, six British citizens are known to have been killed by drones, and previous parliamentary inquiries into the UK government of the legality of such strikes have led to criticism about the vagueness of the UK’s position on combat rules in areas outside armed conflict. There are also doubts about the extent to which the UK military can be legally liable for the specific actions of allies if the allies are acting in concert with the UK.

In October last year, parliament’s joint committee on human rights criticised the government for refusing to explain how targets were selected, whether there was an “impossibility of capture” test, and whether the laws of war were being misapplied outside recognised armed conflicts.

The UK has defended its position by saying strikes are justified if it believes attacks on the UK are imminent. The specific legal advice on which the UK government acts has not been published.