The Israeli military says it has detected “irregular Iranian activity” in Syria and has ordered residents of the occupied Golan Heights to prepare their bomb shelters.
The military says it has prepared for a variety of scenarios.
“Any aggression against Israel will be met with a severe response,” spokesman Jonathan Conricus said.
The Israeli Prime Minister Benjamin Netanyahu said he “fully supports” Mr Trump’s withdrawal.
“The deal didn’t reduce Iran’s aggression, it dramatically increased it and we see this across the entire Middle East,” he said.
Israeli media said it was the first time there had been an order to prepare shelters since the Syrian civil war began, Reuters reported.
Iran has vowed to avenge recent air strikes on its military facilities in Syria that were attributed to Israel.
Israel has neither confirmed nor denied carrying out the strikes, but it has said it will stop what it considers Iran’s military “entrenchment” in Syria.
Israel is also thought to have bombed arms shipments intended for Lebanon’s Hezbollah movement – supported by Iran – several times since the Syrian conflict began.
Iran has backed President Assad throughout Syria’s seven-year civil war, deploying hundreds of military advisers and thousands of militiamen to the country.
Israel seized the Golan Heights from Syria in the closing stages of the 1967 Six-Day War and unilaterally annexed it in 1981. The move is not recognised internationally.