Israel has insisted the assassination of a Hamas leader in Beirut was not an attack on Lebanon, as its enemies warned of "punishment" for his death.
Israel has neither confirmed nor denied that it killed Saleh al-Arouri, but a spokesman called it a "surgical strike against the Hamas leadership".
Hamas denounced it as a "terrorist act", while its ally Hezbollah said it was an assault on Lebanese sovereignty.
Lebanon's PM accused Israel of trying to "drag" it into a regional war.
Lebanese media report that Arouri, a deputy political leader of Hamas, was killed in a drone strike in southern Beirut on Tuesday along with six others - two Hamas military commanders and four other members.
He was a key figure in the Izzedine al-Qassam Brigades, Hamas's armed wing, and a close ally of Ismail Haniyeh, the Hamas leader. He had been in Lebanon acting as a connection between his group and Hezbollah.
There have been near daily exchanges of fire between Hezbollah and Israeli forces since the start of the war between Israel and Hamas in Gaza, but so far the violence has been limited to the area along the Israel-Lebanon border.
Hezbollah - which, like Hamas, is considered a terrorist organisation by Israel, the UK and others - is the largest political and military force in Lebanon and has ministers in the country's government.