Israeli air and naval strikes continued to pummel Gaza as the UK foreign secretary, David Lammy, reiterated his demand for a ceasefire during a visit to Jerusalem.

Strikes on central Gaza followed two days of particularly deadly attacks including one in a humanitarian zone in southern Gaza that killed at least 90 people when Israeli forces targeted the head of Hamas’s military wing, Mohammed Deif.

Hamas has maintained that Deif survived the attack despite public speculation among Israeli officials, but the attempt has further strained already fragile ceasefire negotiations that have dragged for months. “There is no doubt that the horrific massacres will impact any efforts in the negotiations,” the Hamas spokesperson Jihad Taha said on Sunday.

A source close to the negotiations said Qatari mediators remained determined to overcome this latest obstacle, despite the risk that the attempt on Deif’s life could stall talks. They pointed to notable examples where Hamas was reluctant to negotiate but did not disengage entirely, including after an Israeli strike in a Gaza refugee camp last October that killed 120 people, and the assassination of the founder of Hamas’s military wing, Saleh al-Arouri, in Beirut earlier this year.

“Talks still continued then, and they’re going to continue regardless of whether one side wants to take a step back and review,” they said. A second track of negotiations to avert a war between Israel and Lebanon, they added, appeared to be proving more productive for mediators including the White House.

Negotiators from Israel’s Mossad security agency have been engaged in indirect talks with Hamas, mediated by Qatar, Egypt and the head of the CIA, in an attempt to secure the release of dozens of Israeli and dual-national hostages held by Hamas and other Palestinian factions in Gaza since 7 October.

After an initial hostage release in November, efforts to secure a second round in exchange for at least a temporary pause in fighting have proved far more challenging. Israel’s hostages and missing families forum said signs of life had been received from 33 hostages in late May, according to Amnesty International, out of an estimated 116 believed held in Gaza.

Hamas and Israeli officials remain at odds, sometimes down to the exact wording of the truce agreement. The Israeli prime minister, Benjamin Netanyahu, has doubled down on his demand that Israeli forces must be permitted to continue fighting in Gaza, while Hamas has long demanded at least a temporary truce.

Gershon Baskin, a longtime Israeli negotiator involved in the hostage exchange deal that freed the Israeli soldier Gilad Shalit, posted on X about the need to “make a deal now”. Israeli negotiators, he said, should secure a deal and show it to the public “so that everyone will know that the prime minister is the one who is blocking the deal”.

“The attempted elimination of Deif, or the elimination of Deif, will not advance the release of the hostages,” he said. “The military pressure of more than nine months only resulted in the killing of hostages and many Palestinian non-combatants.”

Lammy reiterated his call for a ceasefire during his second day of meetings with Israeli officials, including a meeting with the Israeli president, Isaac Herzog, as the new British foreign secretary continues a diplomatic push despite dwindling hopes of an immediate ceasefire.

Lammy met the family of British hostages and later joined Herzog to meet relatives of Tamir Adar, whose body is believed to be being held by Hamas militants in Gaza. “I hope that we see a hostage deal emerge in the coming days, and I am using all diplomatic efforts,” said Lammy. “I hope, too, that we see a ceasefire soon, and we bring an alleviation to the suffering and the intolerable loss of life that we’re now seeing also in Gaza.”

Far-right members of Netanyahu’s cabinet continue to demand the Israeli government refrain from making a deal to end the fighting in Gaza. The finance minister, Bezalel Smotrich, said Monday that he opposed the release of any Palestinian prisoner as part of a ceasefire agreement, saying: “I will not agree to it, a red line must be drawn.”

He added: “I will oppose this even if it ends my political career.”