By Jonathan Landay, Phil Stewart, Gram Slattery and Parisa Hafezi

WASHINGTON/DUBAI, Feb 28 (Reuters) - Israel and the U.S. timed their attack on Iran on Saturday to coincide with a meeting the country’s supreme leader, Ali Khamenei, was holding with top aides, according to two U.S. sources and a U.S. official familiar with the matter.

Israel said Khamenei was killed along with top lieutenants including Ali Shamkhani, the powerful former National Security Council secretary, and Mohammad Pakpour, Islamic Revolutionary Guard Corps (IRGC) commander.

Jubilant Iranian Americans on Saturday took to the streets from Boston to Los Angeles to cheer the death of supreme leader Ayatollah Ali Khamenei and voice hope for a brighter future for their homeland.

In Los Angeles, home to a massive Iranian diaspora, singing and ululating marchers carried flags of shah-era Iran and posters bearing US President Donald Trump's image, with some wearing shirts that said "Free Iran."

The White House on Saturday released a photo of US President Donald Trump and a handful of his top advisors monitoring the progress of joint US-Israeli air strikes on Iran.

Trump has announced that Ayatollah Ali Khamenei was killed in the attacks, but there is no confirmation from Tehran.

The picture shows Trump, in a suit jacket, white shirt and a white "USA" baseball cap, seated with US Secretary of State Marco Rubio and White House Chief of Staff Susie Wiles at a table, with a partially obscured map marked "Operation Epic Fury" behind them.

Feb 28 (Reuters) - Iranian Supreme Leader Ayatollah Ali Khamenei is dead, U.S. President Donald Trump said on Saturday.

"Khamenei, one of the most evil people in History, is dead," Trump said on Truth Social, without elaborating on the source of the information.

(Reporting by Ismail Shakil; Editing by Cynthia Osterman)

President Donald Trump made his case for war against Iran early on Saturday as US and Israeli forces bombed the Islamic republic, saying conflict was required to eliminate "imminent threats" from Tehran.

Iran "rejected every opportunity to renounce their nuclear ambitions," Trump said in a roughly eight-minute video message posted on social media more than an hour after US strikes began.

Feb 28 (Reuters) - Crossings into the Gaza Strip, vital for the delivery of humanitarian aid and the movement of patients in need of medical evacuation, were closed on Saturday as Israeli and U.S. forces attacked Iran, the Israeli government agency COGAT said.

The closures included the Rafah crossing, located at the Palestinian territory's southern border with Egypt, which was only reopened at the beginning of February to allow a trickle of Palestinians to cross for the first time in months, including patients in need of urgent medical care.

Feb 28 (Reuters) - Iran's Supreme Leader Ayatollah Ali Khamenei, who turned the country into a powerful anti-U.S. force and extended its military influence across the Middle East while crushing repeated domestic unrest, was killed in Israeli and U.S. strikes, a senior Israeli official told Reuters on Saturday.

There was no immediate Iranian confirmation of his fate.

By Samia Nakhoul

DUBAI, Feb 28 (Reuters) - Thunderous blasts and towering fireballs from Iranian missiles streaking across Gulf states vindicated their leaders' long-held fears that Tehran can bring war to their doorstep, likely to harden Arab rulers' support for U.S.–Israeli strikes.

JERUSALEM, Feb 28 (Reuters) - Israeli Prime Minister Benjamin Netanyahu said on Saturday that there were many signs suggesting Iranian Supreme Leader Ayatollah Ali Khamenei may have been killed in Israeli-U.S. strikes in Iran.

"This morning we destroyed the compound of the tyrant Khamenei," Netanyahu said in a video statement, adding that for more than 30 years Khamenei had "dispatched terrorism across the world, made his own people miserable, and worked constantly and tirelessly on a program to annihilate the state of Israel".

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By Alexander Cornwell

TEL AVIV, Feb 28 (Reuters) - Israeli opposition leader Yair Lapid said on Saturday that he backed Prime Minister Benjamin Netanyahu's decision to strike Iran, describing the campaign as a "just war against evil"

The U.S. and Israel launched a series of strikes on Iran, targeting senior leaders and calling for the overthrow of its government, sparking a wave of Iranian missile barrages fired at Israel and Gulf countries including the United Arab Emirates and Qatar.