Israel fired missiles toward Syria's central province of Homs on Sunday evening, targeting an airbase and positions of Iran-backed fighters, an opposition war monitor reported.
The Syrian Observatory for Human Rights, which is based in the UK, said strong explosions were heard when four Israeli missiles hit the Shayrat airbase in Homs province.
It said the missile attack targeted the positions of Iran-backed fighters in the area.
Official Syrian regime news agency SANA said air defences were responding to "hostile targets" over Homs province, adding that some missiles were shot down. It gave no further details.
Syrian TV said "an Israeli aggression" was targeting the province, adding that air defences were responding to the attacks.
The strikes occurred after Israeli warplanes were seen flying over neighboring Lebanon whose airspace Israeli air forces sometimes cross to carry out attacks on Syria.
Israel has carried out hundreds of strikes on targets inside regime-controlled parts of Syria in recent years, but rarely acknowledges or discusses such operations.
Israel has acknowledged, however, that it targets bases of Iran-allied militant groups, such as Lebanon's Hezbollah, which has sent thousands of fighters to support Syrian regime President Bashar Al-Assad's forces.