U.S. President Joe Biden said on Thursday the killing of Palestinian Islamist group Hamas' leader Ismail Haniyeh was not helpful for reaching a ceasefire in Israel's war in Gaza.

There has been an increased risk of an escalation into a broader Middle East war after the assassination of Haniyeh in Iran drew threats of retaliation against Israel.

Hamas and Iran's Revolutionary Guards confirmed the death of Haniyeh, who had participated in internationally-brokered indirect talks on reaching a ceasefire in Gaza.

Anxious residents in Israeli-besieged Gaza feared that Haniyeh's killing on Wednesday would prolong the war.

Iran said the killing took place hours after he attended a swearing-in ceremony for its new president.

"It doesn't help," Biden told reporters late on Thursday, when asked if Haniyeh's assassination ruined the chances for a ceasefire agreement.

Biden also said he had a direct conversation with Israeli Prime Minister Benjamin Netanyahu earlier on Thursday.

Netanyahu's government has issued no claim of responsibility but he has said Israel had delivered crushing blows to Iran's proxies of late, including Hamas and Lebanon-based Hezbollah, and would respond forcefully to any attack.

Israel's tensions with Iran and Hezbollah have fanned fears of a widened conflict in a region already on edge amid Israel's assault on Gaza which has killed tens of thousands and caused a humanitarian crisis.

The latest bloodshed in the decades-old Israeli-Palestinian conflict was triggered on Oct. 7 when Palestinian Islamist group Hamas attacked Israel, killing 1,200 and taking about 250 hostages, according to Israeli tallies.

The Gaza health ministry says that since then Israel's military assault on the Hamas-governed enclave has killed nearly 40,000 Palestinians while also displacing nearly the entire population of 2.3 million, causing a hunger crisis and leading to genocide accusations that Israel denies.

The United States has said it was not involved in the killing of Haniyeh.