He stated the RAF strike that killed two British Islamic country jihadists was a "perfectly prison act of self defence".
There are "other terrorists" concerned in "other plots that can come to fruition" in weeks or months, he said.
Meanwhile, the father of two extra Britons idea to be in Syria stated he believed they had been now on a "hit listing".
Ahmed Muthana, father of Nasser, 20, and Aseel Muthana, 17, from Cardiff, said he notion the pair could be killed in a similar drone strike due to the fact the United Kingdom authorities is "concentrated on anyone now".
Earlier, Mr Fallon might now not be drawn on whether the UK had a "hit list".
'Barbaric assaults'
but he told the BBC: "There are a big quantity of people - now not all British - obtainable in Syria... Who are actively involved in making plans armed assaults right here in Britain."
BBC safety correspondent Frank Gardner stated if this kind of listing existed, it "would be something drawn up digitally", and based totally on intercepting on-line communications, or human informants.
A debate over the UK's use of drones began after MPs have been told on Monday that Cardiff-born Reyaad Khan, 21, had been killed in a precision strike in Raqqa with the aid of a remotely piloted aircraft.
Ruhul Amin, 26, from Aberdeen, became additionally killed within the attack on 21 August. The RAF strike become the first targeted united kingdom drone assault on a British citizen, and was based totally on records gleaned from the United Kingdom's mystery intelligence offerings, the BBC knows.
Mr Fallon said there had been "no different way" of preventing Khan, whom high Minister David Cameron accused of having planned "barbaric" assaults on "excessive-profile public commemorations" in Britain.
"We would not hesitate to take comparable action again," Mr Fallon advised BBC Radio 4's today programme.
By means of Steve Swann, BBC news
In June 2014, Reyaad Khan spoke to me in a web exchange on the day the "there may be no lifestyles without Jihad" video become posted, telling me: "The complete international can sense us marching."
He said he had no longer left his circle of relatives in Cardiff to pursue reputation.
In earlier messages, he were eager to explore the function of overseas coverage in radicalising British Muslims, complaining that "the Syrian issue is being used and brandished about with out explanation by means of coverage makers as justification for further policing legal guidelines towards Muslims inside the united kingdom".
But an increasing number of the messages revealed how an irritated, articulate younger guy, to begin with scathing approximately the view that Britons preventing in Syria posed a risk to the UK, had end up a whole lot more risky.
MPs rejected army motion in Syria two years ago, but Mr Cameron said the legal professional preferred had agreed there was a "clean legal basis" for the RAF strike.
He said it have been accredited at a assembly of "the most senior members" of the national protection Council and accepted with the aid of Mr Fallon.
The prime minister's reputable spokesman said the choice were taken "a few months ago".
Labour questions
The authorities has justified its choice to launch the strike below Article 51 of the United international locations charter, which says member states have an "inherent right of self-defence" if an armed assault is happening or is thought to be drawing close.
But performing Labour leader Harriet Harman and the SNP's Westminster chief Angus Robertson have called for the Intelligence and protection Committee (ISC) to research. The ISC can summon intelligence chiefs to present proof, but a new committee has not but been appointed following the overall election.
Labour management candidate Andy Burnham said it became "unacceptable" for ministers to say that they'll no longer submit any further information.
In other tendencies:
circle of relatives friends of Reyaad Khan, along with Muslim leaders in Cardiff, have known as for evidence of his involvement in a planned assault on the UK
One friend of Khan's, referred to as Sajid, advised the BBC that "no Muslim condones being a part of IS"
Ibrahim Alwawi, an imam in Aberdeen, said the dying of Ruhul Amin changed into a unhappy stop because he could have been a successful British Muslim rather than a risk to the united states of america
On Monday, SNP Westminster leader Angus Robertson complained of no longer having been briefed about the British drone deaths, beforehand of David Cameron announcing the information in the Commons. But, it has on the grounds that emerged that SNP leader Nicola Sturgeon became told
every other British fighter, Junaid Hussain, was killed with the aid of a US drone strike on 24 August and the BBC has seen proof that indicates he became concerned in an lively plot in opposition to targets inside the uk.
Hussain is said to had been worried in encrypted cellphone conversations with any other British guy - who can not be named for criminal reasons - who was allegedly at an advanced stage of planning an assault in the united kingdom.
It's miles understood the plot became disrupted before Hussain became killed.
In 2013, MPs rejected uk army movement against President Bashar Assad's regime in Syria, however remaining September accepted British participation in air strikes towards IS objectives in Iraq handiest.
But officials said the United Kingdom would "act right away and give an explanation for to Parliament afterwards" if there was "a critical British country wide interest at stake".
Mr Fallon said a sparkling Commons vote would be wanted for pre-deliberate military movement towards IS in Syria.
Over the last 4 years, more than two hundred,000 Syrians have lost their lives as what commenced as anti-authorities protests escalated into full-scale civil battle.
