By Jasper Ward and Michelle Nichols

UNITED NATIONS United Nations Secretary-General Antonio Guterres said on Monday he was "deeply distressed" by an announcement that Israel will initiate a complete siege of the Gaza Strip following an attack by the Palestinian Islamist group Hamas over the weekend.

"I am deeply distressed by today's announcement that Israel will initiate a complete siege of the Gaza Strip, nothing allowed in – no electricity, food, or fuel," Guterres said.

The Israeli military said it had called up an unprecedented 300,000 reservists and was imposing a total blockade of the Gaza Strip in response to Hamas attacks that killed hundreds of Israelis and prompted retaliatory Israeli air strikes.

Guterres said the situation in Gaza was "extremely dire" before the surprise assault by the militant group, adding that it will "only deteriorate exponentially."

While noting that medical equipment, food and other humanitarian supplies and personnel are needed, Guterres said that relief and entry of essential supplies into the area must be facilitated.

"I urge all sides and the relevant parties to allow United Nations access to deliver urgent humanitarian assistance to Palestinian civilians trapped and helpless in the Gaza Strip," he said while appealing to the international community to mobilize immediate humanitarian support.

Hamas gunmen burst across the fence from Gaza on Saturday, killing Israeli soldiers and civilians and taking dozens of hostages back to the coastal enclave. Israel responded with its heaviest bombardment of Gaza and could be contemplating a ground assault on the territory it withdrew from nearly two decades ago after 38 years of occupation.

The "abhorrent attacks" by Hamas have left 800 Israelis dead and more than 2,500 injured, according to Guterres, who noted alarming reports of more than 500 Palestinians killed and over 3,000 injured as a result of Israeli air strikes.

(Reporting by Jasper Ward and Michelle Nichols; Editing by Paul Simao)