The killing of more than a hundred Palestinians at a food distribution site in Gaza where Israeli troops opened fire, triggering panic as civilians were gathering around food aid trucks, has added urgency to ceasefire talks – but could also complicate them, President Joe Biden told reporters on Thursday.

“We’re checking that out right now, there are two competing versions of what happened. I don’t have an answer yet,” the president told CNN’s Arlette Saenz at the White House.

Asked if he was worried the deaths would complicate negotiations, he responded: “Oh, I know it will.”

At least 100 people were killed at the distribution site after Israeli forces began shooting while civilians were waiting for food, the Ministry of Health in Gaza said. A local journalist said many of those killed were crushed under aid trucks that were trying to escape the gunfire.

The Israel Defense Forces said “the incident is under review.”

Almost five months after Israel’s war against Hamas began, the humanitarian crisis in Gaza is continuing to escalate, with more than 30,000 people killed – including thousands of children – and hundreds of thousands starving as officials warn the territory is on the brink of famine after relentless Israeli ground and air campaigns.

While Israel faces mounting global pressure to halt the conflict, it has largely retained the support of the United States. President Joe Biden has resisted calls to request a permanent ceasefire.

Biden’s reticence to break with Israeli Prime Minister Benjamin Netanyahu threatens to cost him politically. While the president handedly won Tuesday’s Democratic presidential primaries in Michigan, more than 100,000 Michiganders voted “uncommitted” after a push from Arab-American leaders to show voters’ discontent with Biden’s Israel policy. Michigan will be a crucial state for Biden in the general election.

There has been progress in ceasefire negotiations in recent weeks but a senior Hamas leader warned Thursday’s killings could lead to the talks’ failure.

Biden spoke on Thursday with the leaders of Qatar and Egypt – countries playing an important role in negotiations, a White House official said.

The situation “gives even added urgency to the process,” a senior administration official told CNN.

In Biden’s conversation with Qatari Emir Sheikh Tamim Bin Hamad Al-Thani, both leaders “grieved the loss of civilian lives and agreed that this incident underscored the urgency of bringing negotiations to a close as soon as possible and expanding the flow of humanitarian assistance into Gaza,” the White House said in a readout.

The call with President Abdel Fattah Al-Sisi of Egypt was similar, according to the White House readout. Biden also thanked Al-Sisi for his “leadership in delivering humanitarian aid to Gaza.”

For weeks, senior members of Biden’s national security team have been intimately involved in facilitating negotiations between Israel and Hamas that would prompt a weeks-long pause in fighting and the release of Israeli hostages held by Hamas. It would mark the first cessation in the war since late November.

Biden himself said this week that he was hopeful a ceasefire could be in place by next Monday – a statement that surprised some of the parties involved in the talks. The White House said Wednesday that the president continues to feel “optimistic” about that timeline.

A spokesperson for the National Security Council said the White House is looking into what happened, calling it “serious.”

“We mourn the loss of innocent life and recognize the dire humanitarian situation in Gaza, where innocent Palestinians are just trying to feed their families. This underscores the importance of expanding and sustaining the flow of humanitarian assistance into Gaza, including through a potential temporary ceasefire. We continue to work day and night to achieve that outcome,” the spokesperson said.

Biden on Thursday continued to express optimism that a deal on the hostages and a potential ceasefire could be reached soon, but perhaps not as soon as he originally hoped.

“Hope springs eternal,” Biden said. “I was on the telephone with people in the region, I’m still – probably not by Monday but I’m hopeful.”

This story has been updated with additional developments.