Israeli Prime Minister Benjamin Netanyahu says the country is in a "multi-front war" against Iran and its proxies, following the assassination of a top Hamas leader in Iran.

Netanyahu on Sunday described Iran and Iranian-backed groups as "Iran's axis of evil," and warned that Israel will "exact a heavy price for any act of aggression."

Tensions have been running high in the Middle East since Hamas political leader Ismail Haniyeh was killed last Wednesday during a visit to Iran. Iran is set to retaliate, saying he died in an Israeli attack.

The Iran-backed armed groups, including Lebanon-based Shia Muslim group Hezbollah, may also join the retaliation. A senior Hezbollah commander was killed in an Israeli airstrike.

Israel and Hezbollah continued to exchange fire. The Israeli military said on Monday that it carried out an airstrike on Hezbollah targets in response to a drone attack from Lebanon.

Amid escalating tensions, the Japanese government has raised its travel alert for all of Lebanon to the highest level of its four-tier system, which urges all Japanese nationals to evacuate immediately.

The United States and Britain are also urging their citizens to leave.

US news website Axios quoted multiple sources as saying US Secretary of State Antony Blinken told his Group of Seven counterparts on Sunday that retaliatory attacks could begin as early as the next 24 to 48 hours.

Some Israeli media outlets say that Israel is considering launching a preemptive strike if it uncovered airtight evidence that there is going to be an attack against Israel from a certain place.