An Israeli strike hit Beirut's southern suburbs on Monday as Israel's army said it was targeting Hezbollah, with the raid sending a large plume of smoke billowing across the skyline.

Israeli strikes in Lebanon on Sunday killed at least 15 people, a day after Israel threatened to hit Lebanon's main border crossing with Syria, forcing it to close.

Lebanese Christians marked Easter Sunday by turning their prayers to the south, where villages remain trapped by fighting between Israel and Hezbollah.

The United Nations Interim Force in Lebanon said attacks by Israel and Hezbollah near its positions "could potentially draw return fire".

Iran's Revolutionary Guards said they are completing preparations to enforce new operating conditions in the Strait of Hormuz, which has been all but shut since the war with the United States and Israel began.

U.S.-Israeli strikes killed on Monday the intelligence chief of Iran's Revolutionary Guards, the Guards said.

Iran launched fresh attacks across the Middle East on Monday and threatened "devastating" retaliation, after U.S. President Donald Trump's expletive-laced warning that Tehran must reopen the Strait of Hormuz.

An Israeli strike targeted an apartment in a residential building in Ain Saadeh on Sunday, a town east of Beirut in the hills overlooking the city, the state-run National News Agency reported.

The area had until now been spared clashes between Hezbollah and Israel, which on Sunday stepped up its strikes on the Lebanese capital's southern suburbs, including an attack that killed four people near Beirut's main public hospital.

Israel's military chief Eyal Zamir visited troops in southern Lebanon on Sunday and pledged to intensify strikes against Hezbollah.

U.S. President Donald Trump on Sunday appeared to extend by 24 hours his deadline for Iran to make a deal to reopen the Strait of Hormuz or face devastating infrastructure attacks.