GEOFF BENNETT: Welcome to the "News Hour."

The top political leader of Hamas, Ismail Haniyeh, was killed overnight in Tehran just hours after an Israeli airstrike killed one of the top military leaders of Hezbollah and Beirut.

Taken together, after 10 months of war in Gaza, the attacks escalated fears that a simmering regional conflict could explode into a new and more dangerous phase.

Nick Schifrin starts our coverage.

NICK SCHIFRIN: In the hours before his death, Hamas political leader Ismail Haniyeh met Iranian Supreme Leader Ayatollah Ali Khamenei, Iran's new president, and with Iranian lawmakers pledged the victory he spent his life promising.

It was his final public expression.

Overnight, he and his Iranian bodyguard were killed in what Hamas and Iran described as an Israeli drone strike.

Iran immediately vowed vengeance.

Khamenei posted: "It is our duty to take revenge."

And the Islamic Revolutionary Guard Corps said Israel will -- quote -- "face a harsh and painful response from the powerful and huge resistance front, especially Islamic Iran."

That threat of regional war was repeated today by Iranian proxies in Yemen, the Houthis, and in Lebanon by Hezbollah and Hamas, which vowed to take the battle with Israel to -- quote -- "new dimensions."

But it was met with defiance by Israeli Prime Minister Benjamin Netanyahu.

BENJAMIN NETANYAHU, Israeli Prime Minister (through translator): We are prepared for any scenario and will stand united and determined against any threat.

Israel will exact a heavy price from any aggression against us on any front.

Never again is now.

(CHEERING) (APPLAUSE) NICK SCHIFRIN: And one week after his address to Congress, Netanyahu today suggested defying the U.S. led to Israel's military successes.

BENJAMIN NETANYAHU (through translator): All the achievements that we have made in recent months we achieved because we did not give in, because we made courageous decisions, despite the great pressure at home and abroad.

NICK SCHIFRIN: Israel didn't make any formal statement about Haniyeh today, but Defense Minister Yoav Gallant visited the Arrow missile defense system designed to protect from Iranian missiles.

YOAV GALLANT, Israeli Defense Minister (through translator): Your actions give us the confidence and space to make decisions.

We do not seek war, but we are preparing for all possibilities.

NICK SCHIFRIN: Haniyeh rose through Hamas' ranks and was an early advocate of Hamas' entering politics.

He became Hamas' prime minister after it won 2006 parliamentary elections.

He nurtured Hamas' connection with and support from Iran.

After handing power over to current Hamas leader and October 7 mastermind Yahya Sinwar, he moved to Qatar, where most recently he led Hamas in ongoing cease-fire talks.

Those are hosted by Qatari Prime Minister Sheik Mohammed bin Abdulrahman Al Than, who asked today: "How can mediation succeed when one party assassins the negotiator on the other side?"

ANTONY BLINKEN, U.S. Secretary of State: This is something we were not aware of or involved in.

NICK SCHIFRIN: In Asia today, Secretary of State Antony Blinken called cease-fire talks the priority.

ANTONY BLINKEN: The imperative of getting a cease-fire, the importance that has for everyone remains.

And we will continue to labor that for as long as it takes to get there.

NICK SCHIFRIN: But Defense Secretary Lloyd Austin also promised to defend Israel from Iranian and Hezbollah attacks.

LLOYD AUSTIN, U.S. Secretary of Defense: If Israel is attacked, we certainly will help defend Israel.

You saw us do that in April.

You can expect to see us do that again.

But we don't want to see any of that happen.

We're going to work hard to make sure that we're doing things to help take the temperature down and address issues through diplomatic means.

NICK SCHIFRIN: But across the region, diplomats tell PBS "News Hour" diplomacy feels a long way off.

Hezbollah was already poised to respond to Israel's assassination also yesterday of its most senior military officer, Fuad Shukr.

Hezbollah today confirmed his death and posted new photos of Shukr next to Hezbollah and Iran's most senior officials.

And so the risk tonight is that a war in Gaza the U.S. was already trying to end would instead spread.

For the PBS "News Hour," I'm Nick Schifrin.