'Destroy and eliminate' Britons fighting with IS: UK minister
"Quite simply, my view is a dead terrorist can't cause any harm to Britain," he told the Daily Mail.
"We should do everything we can do to destroy and eliminate that threat," he added.
An estimated 800 British passport holders travelled to fight in Iraq and Syria, with 130 killed and around 400 returning, leaving 270 still in the Middle East.
The issue came into sharp focus in 2014 when British citizen Mohammed Emwazi – also known as "Jihadi John" – appeared in IS propaganda videos showing the beheading of a number of captives.
Williamson, who became defence chief last month after Michael Fallon resigned over allegations of misconduct, vowed that fighters who fled to other countries would also be tracked and refused passage back to Britain.
"We have got to make sure that as (they) splinter and as they disperse across Iraq and Syria and other areas, we continue to hunt them down," he told the Mail.
"Make sure there is no safe space for them, that they can't go to other countries preaching their hate, preaching their cult of death."
His interview came hours after two men appeared in London court charged with a plot to kill British Prime Minister Theresa May.
Meanwhile, Russia's defence ministry said its mission to oust Islamic State militants from Syria had been "accomplished" with the country "completely liberated" from the extremist group.
"The Russian armed forces' goal to defeat armed groups of the ISIL terrorist organisation in Syria has been accomplished," said senior military officer Sergei Rudskoi, using an alternative acronym for the group.
"There is not a single village or district in Syria under the control of ISIL. The territory of Syria has been completely liberated from fighters of this terrorist organisation," he told reporters.
There has been an "unprecedented" involvement by Russia's airforce in recent days, he said, with warplanes making 100 sorties and staging up to 250 strikes daily.
At the same time, special forces were active on the ground directing planes and "destroying the most odious leaders of militant groups behind enemy lines," he said.
But on Thursday the Syrian Observatory for Human Rights, which monitors the conflict, said that IS still holds about eight percent of Deir az-Zour province.
Rudskoi said "separate sabotage bands of ISIL" could still be operating but would be fought by Syrian government troops, indicating that Russia's involvement would be scaled down.
"With the liquidation of armed bands of the ISIL terrorist group in Syria, the Russian contingent will concentrate its main efforts on providing aid to the Syrian people in rebuilding peace" and ensuring ceasefire commitments were met, he said.
Russia began its bombing raids in September 2015 in support of Syrian President Bashar al-Assad's beleaguered forces.
The controversial strikes, which have been blamed for the deaths of civilians, have helped Assad regain control over much of war-ravaged Syria.
Last month, President Vladimir Putin said efforts to end the war were entering a "new stage" as the focus shifts from military intervention to political reform.
More than 340,000 people have been killed since the conflict broke out in March 2011 with protests against Assad's rule that sparked a brutal crackdown.
Similar victories have been celebrated across the border in Iraq after Iraqi forces and local tribal fighters captured the last IS-held town in the country last month, leaving just a slither of territory in the militants' hands.
Agencies contributed to this report