Israeli Defense Minister Yoav Gallant Monday ordered a "complete siege" on the Gaza Strip as the military pounded the Palestinian enclave with air strikes.

"We are putting a complete siege on Gaza... No electricity, no food, no water, no gas -- it's all closed," Gallant said in a video statement, referring to the crowded enclave home to 2.3 people.

Hamas gunmen killed around 250 people who attended an outdoor music festival in an Israeli community near Gaza at the weekend, a volunteer who helped collect the bodies said on Monday.

"In the area where the party took place, and at the party itself" it could be estimated that "there were 200-250 bodies," said Moti Bukjin, a spokesman for the humanitarian NGO Zaka, based on the number of trucks that ferried away the corpses.

Iranian security officials helped plan Hamas’ Saturday surprise attack on Israel and gave the green light for the assault at a meeting in Beirut last Monday, the Wall Street Journal quoted “senior Hamas and Hezbollah members” as saying.

Anti-rocket sirens followed by the sound of blasts were heard Monday in Jerusalem and Tel Aviv, as Israel's army battled Hamas militants in southern towns for the third consecutive day.

Palestinian groups said they have fired a new barrage of missiles at Jerusalem and Tel Aviv, saying it comes in response to attacks on civilians in Gaza.

With its surprise attack against Israel, Hamas has violently shifted the world's eyes back to the Palestinians and dealt a severe blow to momentum to secure a landmark U.S.-brokered deal between Israel and Saudi Arabia.

The Iranian-backed Islamist militants who run the impoverished, blockaded Gaza Strip on Saturday fired thousands of rockets and infiltrated forces into Israel, 50 years after Arab states' assault on Israel during the Jewish holy day of Yom Kippur.

Israel's army declared on Monday that its forces were in "control of communities" in its southern territory near Gaza, two days after Hamas militants launched a surprise attack there.

"We are in full control of the communities," military spokesman Daniel Hagari told journalists, adding however that there still might be "terrorists" in the area.

The surprise assault by Hamas against Israel was a meticulously planned offensive that the Palestinian militant group is capable of keeping up, with a risk of even greater escalation, analysts say.

Hamas can count on a deep arsenal of rockets to use against Israel but key questions include how much support it has received from Iran, which has expressed its backing for the offensive, and whether Hezbollah will enter the fray.

Iran on Monday rejected as unfounded allegations it had a role in the massive assault on Israel by the Palestinian Islamist group Hamas.

"The accusations linked to an Iranian role... are based on political reasons," foreign ministry spokesman Nasser Kanani told reporters, adding that Palestinians had "the necessary capacity and will to defend their nation and recover their rights" without any help from Tehran.

Israel's defense minister ordered Monday a "complete siege" on the Gaza Strip following an unprecedented incursion into Israel by Hamas fighters.

Israel formally declared war on Sunday and gave the green light for "significant military steps" to retaliate against Hamas for Saturday's surprise attack. More than 1,100 people have been killed and thousands wounded on both sides.

Defense Secretary Lloyd Austin said he has ordered the Ford carrier strike group to sail to the Eastern Mediterranean to be ready to assist Israel after the attack by Hamas that has left more than 1,000 dead on both sides. Americans were reported to be among those killed and missing.

The USS Gerald R. Ford, the Navy's newest and most advanced aircraft carrier, and its approximately 5,000 sailors and deck of warplanes will be accompanied by cruisers and destroyers in a show of force that is meant to be ready to respond to anything, from possibly interdicting additional weapons from reaching Hamas and conducting surveillance.