Mr Netanyahu’s threats come at the end of his four-day diplomatic tour of Europe during which he lobbied European leaders to take a tougher stance against Iran and in particular follow Donald Trump’s lead of withdrawing from the Iran nuclear deal.
Speaking at the Policy Exchange think tank in London, Mr Netanyahu declared: “Mr Assad’s regime is no longer immune.
“If he fires at us, we will destroy his forces.
“I think there is a new calculus that has to take place and Syria has to understand that Israel will not tolerate the Iranian military entrenchment in Syria against Israel.
“The consequences are not merely to the Iranian forces there but to the Assad regime as well, I think it’s something that he should consider very seriously.”
Mr Netanyahu blamed Assad directly for allowing Iranian troops to become entrenched within Syria and in turn threaten Israel.
He said: “Assad has to consider this: when they waged this horrific civil war, Israel did not intervene.
“Now the war is nearly over, he invites Iran in to entrench itself?
Pointing to the danger posed by Iranian forces, he added: “They want to bring in precision-guided rockets with half a ton of warheads and submarines to the Mediterranean.
“Iran does not believe in a master race, it believes in a master faith.”
Mr Netanyahu said his European trip was aimed at exposing how “Iran is devouring one nation after another” and to lobby European governments to do more to “reverse Iran’s aggression in the region”.
He said: “My point was to harness the leading European countries to oppose the continued expansion of Iran.”
However Theresa May rejected calls for the UK to withdraw from the Iran nuclear deal – the Joint Comprehensive Plan of Action (JPCA) – saying: “We will remain committed to it as long as Iran meets its obligations.
“Along with France and Germany the UK continues to believe that is the best route to preventing Iran from getting a nuclear weapon.
“But we do recognise that there are other issues that need to be addressed in relation to Iran – its destabilising regional activity in countries like Syria and Yemen and also the proliferation of ballistic missiles.”
Mr Netanyahu was also criticised at a meeting in London for not using non-lethal weapons to disperse Palestinian protestors at the Gaza border.
But the Israeli Prime Minister responded defiantly: “We have tried all these methods, they don’t work.
“These were not non-violent protests, quite the contrary.
“We are doing everything we can to both minimise casualties and, at the same time, protect Israeli lives.”