Tensions in the Middle East have heightened with Iran and Hamas, along with Hezbollah, vowing strong retaliation to the assassination of their group leaders, blamed on Israel, triggering fears of a regional war.

The killing this week of Hamas chief Ismail Haniyeh in Tehran, hours after the Israeli assassination of Hezbollah's military chief Fuad Shukr in Beirut, has intensified the demand for revenge.

Hezbollah has been exchanging near-daily cross-border fire with Israeli forces since October, claiming to act in support of Hamas.

The group announced it had fired dozens of Katyusha rockets at the northern Israeli settlement of Beit Hillel as Iran said it expects Hezbollah to hit deeper inside Israel and no longer be confined to military targets, media reports said.

The US, UK and France called on their citizens to leave Lebanon - where the powerful Iran-backed Hezbollah movement is based - amid airlines cancelling flights.

Canada has also warned citizens to avoid all travel to Israel, saying the regional armed conflict endangered security.

Meanwhile, Indian nationals in Israel have been asked to stay vigilant and adhere to the safety protocols following the escalating tension in the region.

The advisory for the Indian citizens in Israel comes a day after the Indian Embassy in Beirut strongly advised Indian nationals against travelling to Lebanon till further notice. It also advised them to leave Lebanon.

Haniyeh was Hamas's lead negotiator in efforts to end the war with Israel. His assassination has raised questions about the continued viability of efforts by Qatari, Egyptian and US mediators to mediate a truce and exchange of hostages and prisoners.

Hamas said it had begun a "broad consultation process" to choose a new leader three days after the assassination of Haniyeh.

US President Joe Biden expressed hope Iran would stand down despite its threat to avenge the assassination of Hamas and Hezbollah leaders in Tehran. Biden said he was "very concerned" about rising tensions in the region, adding that the killing of Haniyeh had "not helped" the situation.

Israel, meanwhile, warned its adversaries they would "pay a very high price" for any "aggression". "Israel is at a very high level of preparation for any scenario, both defensive and offensive," Prime Minister Benjamin Netanyahu said in a statement.

The war in Gaza has caused widespread destruction and displaced almost the entire population of the territory, according to the UN statement on Friday. It said public health conditions "continue to deteriorate".