Drones were seen flying low as they passed over Iraq, in an apparent attempt to avoid radar detection.
A loud buzzing noise could be heard on videos posted on social media as they passed overhead.
“We are closely monitoring Iranian killer drones that are en route to Israel sent by Iran,” Daniel Hagari, an IDF spokesman, said in a televised statement.
“This is a severe and dangerous escalation. Our defensive and offensive capabilities are at the highest level of readiness ahead of this large-scale attack from Iran.”
He added: “We are working in close cooperation with the United States and our partners in the region in order to act against the launches and intercept them.”
The Israeli military has already begun planning for a retaliatory strike on Iran or its proxy groups in the region.
Israeli civilians have been advised to monitor government alerts and follow advice from the Home Front command.
Schools across the country and beaches near Gaza have been closed “in light of the security situation”, Hagari said.
He added that the IDF’s interception attempts would scramble GPS signals across the country.
The Israeli defences will involve fighter jets and navy ships, which are both equipped with anti-aircraft missiles and Israel’s Iron Dome missile defence system.