The first of 20 trucks carrying humanitarian aid entered the war-torn and besieged Gaza Strip on Saturday through the Rafah border crossing with Egypt, said AFP correspondents on both sides.

UN humanitarian chief Martin Griffiths said he was "confident that this delivery will be the start of a sustainable effort to provide essential supplies... to the people of Gaza" and warned that "this first convoy must not be the last".

The border crossing was closed again after the passage of the trucks from the Egyptian Red Crescent, which is responsible for delivering aid from various UN agencies, reports AFP.

It was the first such delivery since the war broke out more than two weeks ago between Israel and Hamas, the Islamist militant movement which rules the Palestinian enclave of 2.4 million people.

Rafah is the only route into Gaza that is not controlled by Israel, which agreed to allow the aid in from Egypt following a request from its top ally the United States.

Israel has been bombing Gaza since Hamas' bloody surprise attack of October 7 and has also declared a total siege, cutting off most water as well as food, electricity, fuel and other supplies.

Hamas militants stormed into Israel from Gaza and killed at least 1,400 people, mostly civilians who were shot, mutilated or burnt to death on the first day of the raid, according to Israeli officials.

Since then, more than 4,100 Palestinians, mainly civilians, have been killed in relentless Israeli bombardments, according to Gaza's Hamas-run health ministry.

Cargo planes and trucks have been bringing humanitarian aid to the Egyptian side of Rafah for days, but so far none had been delivered to Gaza.

UN Secretary-General Antonio Guterres on Friday visited the Egyptian side of the crossing to oversee preparations for the aid delivery.

"These trucks are not just trucks, they are a lifeline," he said. "They are the difference between life and death for so many people in Gaza."

The UN chief said it must be "a sustained effort" with not just one convoy crossing but for many "to be authorised in a meaningful number to have enough trucks to provide support to Gaza's people".

"It is essential to have fuel on the other side... to be able to distribute humanitarian aid for the population in Gaza," Guterres said, warning against the use of aid deliveries as "bargaining chips".

Israel worries that any fuel brought into Gaza could be used by Hamas to manufacture weapons and explosives.

US President Joe Biden had pushed for the trucks to be allowed to pass, during a solidarity visit to Israel on Wednesday.

He has said the first 20 trucks will be a test of a system for distributing aid without allowing Hamas to benefit, with UN agencies set to distribute it on the Gaza side of the border.