The prime minister of Israel has declared the country is at “war” after Hamas launched an unprecedented surprise multi-front attack on Saturday morning.

The centre and south of Israel have been under sustained rocket fire for more than three and a half hours according to Israeli media. Earlier armed gunmen reportedly infiltrated Israeli communities after crossing the border fence.

Benjamin Netanyahu released a video address responding to the attack in which he said: Citizens of Israel, we are at war. Not an operation, not an escalation — at war.” Thousands of reservists are being called up.

The Israeli Defence Forces has declared a “state of readiness for war” and vowed that “Israel will win” after Hamas launched a surprise attack on Israel this morning.

Hamas has so far taken 35 Israeli hostages, according to Israel media. At least 40 Israelis are thought to have been killed and more than 545 injured, according to the health ministry. The major attack represents one of Israel’s most serious security blunders.

Defense Minister Yoav Gallant warned that Hamas “made a grave mistake” in one of the most serious escalations of the conflict in years. The Israeli Air Force has begun hitting the Gaza Strip and fighting between troops and militants is ongoing in at least eight locations.

“Today the people are regaining their revolution,” leader of Hamas’ military wing Mohammed Deif said in a recorded message, as he called on Palestinians from east Jerusalem to northern Israel to join the fight and “expel the occupiers and demolish the walls.”

Israel also appears to have intelligence that suggests a possible attack from Hezbollah militants in Lebanon. “We’re hearing noises from the north, and we’re prepared,” IDF spokesperson Richard Hecht told reporters.