A blown-out building in the Lebanese capital flashed on local TV channels as plumes of black smoke billowed into the sky.
Hamas said this was the aftermath of an Israeli strike that killed one of its top officials, Saleh Al-Arouri, the deputy head of the group's political bureau, and several other members.
Israel hasn't confirmed or denied responsibility for the attack.
If true, Arouri would be the most senior Hamas official killed by Israel since the start of the war sparked by the October 7 Hamas attack on Israel. The suspected strike would also risk further broadening the arena of the Israel-Hamas conflict.
Israel's stated aim from that war was to eliminate Hamas in Gaza and free the more than 240 hostages the group captured from Israel during its assault.
But Israel has yet to capture or kill the group's top leadership in the enclave – where Israeli strikes have exacted a staggering death toll – and many of the hostages remain in Hamas' captivity.
Tuesday's attack would mark the biggest Israeli strike on the Lebanese capital since the 2006 war between the two countries.
It took place in Beirut's southern suburbs, in a popular neighborhood that is also a stronghold of the Iran-backed militant group Hezbollah, sparking concerns that the war could engulf neighboring countries and risk drawing in regional powers like Iran, as well as the United States.
Lebanese Prime Minister Najib Mikati called it a "new Israeli crime that aims to drag Lebanon into a new phase in confrontations," referring to the ongoing tit-for-tat conflict between Hezbollah and Israeli forces on the Lebanon-Israel border.
For nearly three months, the fighting between Hezbollah and Israel has largely been confined to a roughly four-kilometer (2.5 mile) range of the border region, with Hezbollah striking Israel while Israel struck Lebanon.
At points it seemed to teeter on the precipice of a full-scale war between Israel and the Middle East's most powerful paramilitary, Hezbollah, but fell short of sparking the conflict that the US and other Western countries feared.
That may have changed Tuesday afternoon, as the rules of engagement seemed to suddenly shift with the blast in Beirut.
"Hezbollah's choices narrow down to retaliation or capitulation," Firas Maksad, a senior fellow at Washington DC's Middle East Institute, said on X, formerly Twitter. "Hezbollah's preference is for grey-zone warfare- harassing Israel but short of war. That got more difficult today... Hezbollah will now have to go up the escalation ladder and respond in kind."
Hezbollah has vowed to retaliate. "We consider the crime of the assassination of Sheikh Saleh Arouri and his martyr companions in the heart of the southern suburbs of Beirut to be a dangerous attack on Lebanon, its people, its security and its resistance," the group said in a statement on Tuesday night. "We at Hezbollah confirm that this crime will not pass without a response and punitive measures."
In two speeches delivered since October 7, but before Tuesday's explosion, Nasrallah repeatedly raised the prospect of an all-out war with Israel, but ultimately seemed to suggest that Hezbollah was intent on restricting its fight to the border area.
The fighting has since intensified in the border region and Israeli officials have threatened to ramp up their attacks on Hezbollah.
Mark Regev, a senior adviser to Israeli Prime Minister Benjamin Netanyahu, told MSNBC that Israel had not taken responsibility for Arouri's death.
"But whoever did it must be clear that this was not an attack on the Lebanese state. It was not an attack even on Hezbollah," Regev said. "Whoever did this did a surgical strike against the Hamas leadership."
A US official however told CNN that Israel carried out the strike, adding that the Biden administration was not told about the operation in advance.
The Beirut attack came amid heightened tensions in the region that had already raised the specter of a wider war.
The US has recently stepped up its military involvement in the Middle East. Last month, the military carried out airstrikes on Iraq-based Kataib Hezbollah and "affiliated groups" after an attack injured three US troops.
And last week, US helicopters sank three Houthi boats in the Red Sea after coming under fire, killing those on board. The event marked the first time since tensions broke out earlier last year that the US killed members of the rebel group. The White House said it wasn't seeking a wider conflict.
The Houthis, who condemned the Beirut attack as a "cowardly crime" by Israel, have carried out several attacks on merchant vessels in the Red Sea in retaliation for Israel's assault on Hamas, disrupting trade in one of the world's most important waterways.
Last week, Iran and several of its armed proxies accused Israel of assassinating a senior Iranian commander in Syria, vowing retaliation. Israel didn't comment on the matter.
"This (Beirut attack) shows once more that as time has passed and Biden has refused to push for a ceasefire (in Gaza), we are getting closer and closer to a full war in the region," Trita Parsi, Vice President of the Quincy Institute in Washington DC, wrote on X. "We are seeing escalation in the Red Sea... We are seeing it in Lebanon. And we are seeing it in Iraq and Syria, where militias have targeted US troops... The US will very likely get dragged into a region-wide war."
CNN's Tamara Qiblawi, Alex Marquardt, Abbas Al Lawati, Niamh Kennedy and Hamdi Alkhshali contributed to this report.
