Diplomats across the Middle East worked on Monday to contain escalating tensions between Israel and Iran as fears grew of a widening conflict in the region and Tehran vowed to retaliate for the killing of a senior Hamas leader, Ismail Haniyeh, on Iranian soil.

With Israel’s war with Hamas raging in Gaza, the killing of Mr. Haniyeh last week in Tehran has intensified concerns among Arab and American officials that an even broader regional conflict could erupt. Israel has not publicly taken responsibility for the killing, but Iran and Hamas have both blamed Israel, and American intelligence has assessed that Israel was behind it.

Israel’s defense minister, Yoav Gallant, who was briefed on the country’s air-defense readiness on Monday during a visit to a military command center, said, according to a government statement, “We must be prepared for anything — including a swift transition to offense.”

In a separate statement, the Israeli government said Mr. Gallant had briefed the U.S. defense secretary, Lloyd J. Austin III, overnight on Israel’s “readiness to defend Israel against potential threats posed by Iran and its proxies.”

Mr. Austin on Friday ordered more combat aircraft and missile-shooting warships to the Middle East in response to the threats from Iran and its proxies, the Pentagon said.