Israeli Prime Minister Benjamin Netanyahu said Saturday that Israel is "at war" after Palestinian resistance group Hamas launched an operation from the blockaded Gaza Strip.

"We are at war, and not in a military operation," Netanyahu said in a statement.

"Hamas launched a sudden deadly attack against the State of Israel and its citizens," the Israeli premier said.

He ordered "mobilizing military reserves on a large scale."

"The enemy will pay an unprecedented price," Netanyahu said.

In a brief video address later, he said Hamas would be held responsible for the well-being of the captives and said Israel would settle the score with anyone who harmed them.

He also asked Palestinians to leave Gaza, saying that the Israeli army will turn Hamas sites into rubble.

The Israeli Broadcasting Corporation said that Defense Minister Yoav Galant approved the mobilization of reserve forces after announcing preparations to deal with the situation as a state of war.

Israel has declared a "state of readiness" for war after Palestinian factions in the Gaza Strip fired an intense barrage of rockets early Saturday toward Israel.

The rocket salvoes were fired from different locations in the enclave, bypassing the Iron Dome defence system and hitting buildings.

At least 100 Israelis were reportedly killed and over 1,000 others injured in the Gaza rockets attack, the Israeli media reported.

Sirens sounded in central Israel and Tel Aviv, the Israeli army said in a statement.

Sirens sounded also in Tel Aviv, Rehovot, Rishon Lezion and the Palmachim Airbase, according to media outlets, including the Walla news website.

Schools were suspended in the Gaza Strip until further notice, according to the Palestinian Education Ministry in Gaza.