Syrian media reports say Israeli warplanes have carried out dozens of attacks across the country, including in the capital, Damascus.
The UK-based Syrian Observatory for Human Rights (SOHR) said there were more than 100 strikes on military targets.
A research centre with suspected links to chemical weapon production was among the sites hit, according to local media reports.
Israel says it is acting to stop weapons falling “into the hands of extremists” following the overthrow of the Assad regime.
On Monday, the UN Security Council met to discuss the situation in the country following the downfall of President Bashar al-Assad, and said they will work on a statement in the coming days.
“The council, I think, was more or less united on the need to preserve the territorial integrity and unity of Syria, to ensure the protection of civilians, to ensure that humanitarian aid is coming to the needy population,” Russian UN Ambassador Vassily Nebenzia told reporters.
The SOHR says there have been hundreds of Israeli air strikes in the past two days, including on a site in Damascus said to have been used for rocket development by Iranian scientists.
The strikes come as the UN’s chemical watchdog warns authorities in Syria to ensure that suspected stockpiles of chemical weapons are safe.
According to the UN’s chemical watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW), a chemical weapon is a chemical used to cause intentional death or harm through its toxic properties.
The use of chemical weapons is prohibited under international humanitarian law regardless of the presence of a valid military target, as the effects of such weapons are indiscriminate by nature.
It is not known where or how many chemical weapons Syria has, but it’s believed former President Bashar al-Assad kept stockpiles and that the declaration he had made was incomplete.
Syria signed the OPCW’s Chemical Weapons Certificate in 2013, a month after a chemical weapons attack on suburbs of the capital, Damascus, that involved the nerve agent sarin and left more than 1,400 people dead.
The horrific pictures of victims convulsing in agony shocked the world. Western powers said the attack could only have been carried out by the government, but Assad blamed the opposition.
Despite the OPCW and the UN destroying all 1,300 tonnes of chemicals that the Syrian government declared, chemical weapons attacks in the country still continued.
On Monday, the OPCW said it had contacted Syria “with a view to emphasising the paramount importance of ensuring the safety and security of all chemical weapons related materials and facilities” in the country.
Also on Monday, the Israeli military released photos of its troops who crossed from the Israeli-occupied Golan Heights into the demilitarised buffer zone in Syria where UN peacekeepers are based.
It comes a day after Israeli Prime Minister Benjamin Netanyahu announced that the military had temporarily seized control of the so-called Area of Separation, saying the 1974 disengagement agreement with Syria had “collapsed” with the rebel takeover of the country.
The Golan Heights is a rocky plateau about 60km (40 miles) south-west of Damascus.
Israel seized the Golan from Syria in the closing stages of the 1967 Six-Day War and unilaterally annexed it in 1981.
The move was not recognised internationally, although the US did so unilaterally in 2019.
Speaking at a news conference on Monday, Saar said the Israel Defense Forces (IDF) was only making “a very limited and temporary step” taken for “security reasons”.
He also claimed that Israel had no interest in meddling in internal Syrian affairs and was concerned only with defending its citizens.
Defence Minister Israel Katz meanwhile said the Israeli military would “destroy heavy strategic weapons” – including missile and air defence systems.
