RAFAH, Gaza Strip — At least 31 people were killed and over 170 were wounded Sunday while on their way to receive food in the Gaza Strip, health officials and witnesses said. Witnesses said Israeli forces fired on crowds around 1,000 yards from an aid site run by an Israeli-backed foundation.

It was the deadliest incident yet around the new aid distribution system, Gaza Humanitarian Foundation, which has operated for less than a week.

Israel’s military said its forces did not fire at civilians near or within the site. An Israeli military official, speaking on condition of anonymity in line with procedure, said troops fired warning shots at several suspects advancing toward them a kilometer from the site.

The foundation — promoted by Israel and the United States — said it delivered aid “without incident.” It has denied previous accounts of chaos and gunfire around its sites, which are in Israeli military zones where independent media has no access.

The International Committee of the Red Cross said its field hospital in the southern city of Rafah received 179 casualties, including women and children; 21 were declared dead upon arrival, the majority with gunshot or shrapnel wounds.

“All patients said they had been trying to reach an aid distribution site,” the ICRC said, calling it the highest number of “weapon-wounded” people in one incident since the hospital was set up over a year ago.

“Aid distribution has become a death trap,” the head of the U.N. agency for Palestinian refugees, Philippe Lazzarini, said.

Also, Israeli military chief of staff Lt. Gen. Eyal Zamir ordered that more aid sites be established — and troops’ operation be expanded in unspecified parts of northern and southern Gaza.

Multiple witnesses have said Israeli troops fired on crowds near the new foundation’s sites. Before Sunday, 17 people were killed while trying to reach them, said Zaher al-Waheidi, head of the Gaza Health Ministry’s records department.

The foundation says private security contractors guarding its sites have not fired on crowds. Israel’s military has acknowledged firing warning shots on previous occasions.

Also Sunday, climate campaigner Greta Thunberg and other 11 activists set sail for Gaza on a ship aimed at “breaking Israel’s siege” of the devastated territory, organizers said.

The sailing boat Madleen, operated by activist group Freedom Flotilla Coalition, departed from the port of Catania in Sicily, Italy.

It will try to reach the shores of the Gaza Strip to bring in some aid and raise “international awareness” over the humanitarian crisis, the activists said ahead of their departure.

“We are doing this because, no matter what odds we are against, we have to keep trying,” Thunberg said, bursting into tears during her speech.

“Because the moment we stop trying is when we lose our humanity. And no matter how dangerous this mission is, it’s not even near as dangerous as the silence of the entire world in the face of the livestreamed genocide.”