Israeli Prime Minister Benjamin Netanyahu has told the BBC that Israel has a right to prevent weapons from falling into the wrong hands in Syria.
He said that if terrorists seized anti-aircraft and chemical weapons they could be "game changers" in the region.
There have been growing calls for the international community to arm rebels fighting President Bashar al-Assad.
But there is increasing concern that Islamist militants could use such weapons to further their own causes.
Israel has said its policy is not to get involved in the Syrian conflict.
But in recent months it has retaliated following Syrian firing into Israeli-controlled areas in the Golan Heights.
Israel first occupied the Golan Heights in 1967 and later annexed the territory in a move that is not internationally recognised.
Mr Netanyahu, in an exclusive interview with the BBC's Lyse Doucet, said Israel's concern was "which rebels and which weapons?"
"The main arms of concern to us are the arms that are already in Syria - these are anti-aircraft weapons, these are chemical weapons and other very, very dangerous weapons that could be game changers," he said.
"They will change the conditions, the balance of power in the Middle East. They could present a terrorist threat on a worldwide scale. It is definitely our interest to defend ourselves, but we also think it is in the interest of other countries."
Mr Netanyahu was in London to attend the funeral of former prime minister Baroness Thatcher, and also held talks with current Prime Minister David Cameron.
Asked if Israel would adopt a more aggressive military stance in Syria, Mr Netanyahu said: "We are not aggressive. We don't seek military confrontation, but we are prepared to defend ourselves if the need arises and I think people know that what I say is both measured and serious."
Mr Netanyahu would not confirm what was widely believed to have been an Israeli air strike on a suspected Syrian government weapons convoy in January.
It was reported that the convoy had been heading for Hezbollah fighters in Lebanon.
Mr Netanyahu also reiterated his view that Iran and its nuclear programme posed a direct threat to world peace, which could only be stopped by a "direct military threat", not sanctions or tough diplomacy.
"Other countries, once they see Iran getting nuclear weapons, will rush to get their own nuclear weapons and then the Middle East will become a tinderbox," he said.
He added that the current North Korea crisis had shown world leaders what could happen when a rogue state acquired nuclear weapons.
"The entire world is paralysed [and] destabilised," Mr Netanyahu said.
"Iran is many times stronger than North Korea, both in GDP and aggressive tendencies. I think there's an interesting change of perception because people can understand what it would be like to have Iran with their imperial ambitions, with their messianic and apocalyptic ideology possess atomic bombs."
Asked about the Israeli-Palestinian peace process, Mr Netanyahu said the creation of a two-state solution depended "on the Palestinians".
"I'm ready to sit down. I think we shouldn't talk about the talks. We should just get on with it and try to negotiate a real lasting and defensible peace between us."