The use of a chemical weapon in the internal war in Syria has put the Israeli air force on the highest level of alert, with a "massive array" of sensors now directed at the suspect areas.
According to reports from Syria at least one such weapon was used several days ago, during clashes between the Syrian army and rebel forces. The Syrian government and rebels accused each other of launching the deadly attack, which allegedly took place near the northern city of Aleppo.
The USA has said it is evaluating allegations of chemical weapons use in Syria, but has already dismissed claims that opposition forces had used such weapons during the two-year-old conflict.
Israel has been following moves involving Syria's chemical weapons arsenal in recent months, with its air force having expressed deep concern about the stockpile of materials. Any sign of an intention to use such weapons against Israel will be met with a "fierce reaction", the service adds.
The Syrian army has a large stockpile of weapons of mass destruction, and Israeli intelligence sources have previously said that some chemicals have been added to the warheads of its Scud-D ballistic surface-to-surface missiles.