Saleh al-Arouri was a member of Hamas' political bureau and a co-founder of its military wing.
His killing marks the first assassination of a Hamas official outside Palestinian Territories since the group's deadly assault on Israeli territory on October 7... and could signal the conflict between Hamas and Israel is expanding.
In response to questions from Reuters, the Israeli military said it does not respond to reports in foreign media.
Lebanon's national news agency said the drone struck a Hamas office. Two security sources said it had targeted a meeting and left several people dead, including Palestinians and Lebanese.
Protests broke out in the West Bank following the attack.
Lebanon's caretaker prime minister Najib Mikati called the strike a "new Israeli crime" and said it was an attempt to pull Lebanon into war.
His office said he asked Lebanon's foreign minister to file a complaint to the United Nations Security Council.
A spokesperson for the U.N. secretary-general called the strike "extremely worrying."
"Because of the escalating tensions and the fragility of the situation in the region, we are calling for maximum restraint from all parties. We don't want any rash, any rash actions that could trigger further violence."
Hezbollah has been exchanging near-daily fire with Israel across Lebanon's southern frontier since Hamas' attack in October.
Israeli air strikes and shelling have killed more than 100 Hezbollah fighters and nearly two dozen civilians since then.
In a written statement, Hezbollah said the attack "will not go without a response or punishment." It announced an attack on Israeli forces across the border, but did not specify whether it was in response to Arouri's killing.
In Gaza, Palestinian health officials say the death toll has surpassed 22,000.
