Hamas said Tuesday its deputy head Saleh al-Arouri was killed in a blast in Beirut.
He is said to have been among four people killed in an explosion at Hamas's office in Dahiyeh, one of the Lebanese capital's southern suburbs.
Al-Arouri was the deputy head of Hamas's political bureau and known to be deeply involved in its military affairs.
An explosion shook the Lebanese capital's southern suburbs on Tuesday evening causing chaos in the militant Hezbollah group's stronghold, but the nature of the blast was not immediately known.
It was not clear if the explosion inflicted any casualties in the Beirut suburb but videos circulating on social media showed serious damage and fire.
The explosion came during MORE than two months of heavy exchanges of fire between Israeli troops and members of Hezbollah along Lebanon's southern border.
Since the fighting began it has been concentrated a few miles from the border but on several occasions Israel's air force hit Hezbollah targets deeper in Lebanon.
Earlier in the day, Hezbollah said its fighters carried out several attacks along the Lebanon-Israel border targeting Israeli military posts.
Elsewhere, the Palestinian Red Crescent says five people have been killed and three injured in Israeli bombing on its headquarters in Khan Younis, in south Gaza.
Intense fighting has continued in the territory, even as Israel announced plans to pull back some combat troops.
Meanwhile, the Israeli military says its troops have killed dozens of Palestinian fighters in Jabalia in the north, and targeted others in Gaza City and coastal areas.
At least 1,200 people were killed when Hamas militants attacked Israel on 7 October and about 240 others were taken hostage.
The Hamas-run health ministry in Gaza says MORE than 22,000 people have been killed in the territory since Israel started its retaliatory campaign.
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