By Michelle Nichols

UNITED NATIONS (Reuters) - The Palestinian envoy to the United Nations on Tuesday described Israel's bombardment of the Gaza Strip and vow to impose a complete siege on the Hamas-controlled Palestinian enclave as "nothing less than genocidal."

Hamas militants carried out their deadliest attack in Israel's history on Saturday, when gunmen rampaged through Israeli towns, killing more than 1,000 people and taking scores of hostages to Gaza. Israel has retaliated with air strikes on Gaza that have razed entire districts as it prepares for a possible ground offensive.

Israel's Defence Minister Yoav Gallant drew international condemnation by announcing on Monday a "total blockade" to stop food and fuel reaching Gaza, home to 2.3 million people. Gallant said Israel was battling "beastly people."

"Such blatant dehumanization and attempts to bomb a people into submission, to use starvation as a method of warfare, and to eradicate their national existence are nothing less than genocidal," Palestinian U.N. envoy Riyad Mansour wrote in a letter to the U.N. Security Council on Tuesday, seen by Reuters.

"These acts constitute war crimes," he wrote.

U.S. national security adviser Jake Sullivan said on Tuesday that his understanding was that "the concept of siege is not something that in fact is going to be pursued by the Israeli government," adding that Washington was speaking with the Israeli government "about their actions in this regard."

Sullivan also said U.S. President Joe Biden and Israeli Prime Minister Benjamin Netanyahu on Tuesday discussed "the difference between going full bore against Hamas terrorists and how we distinguish between terrorists and innocent civilians."

Israel's U.N. Ambassador Gilad Erdan on Sunday accused Hamas of war crimes and said it was time to "obliterate Hamas terror infrastructure," as the 15-member U.N. Security Council met behind closed-doors on the conflict.

U.N. Secretary-General Antonio Guterres on Monday said he was "deeply distressed" by Israel's announcement of a complete siege on Gaza.

"The humanitarian situation in Gaza was extremely dire before these hostilities; now it will only deteriorate exponentially," Guterres said.

(Reporting by Michelle Nichols; editing by Grant McCool)