As Iran threatens to respond to the suspected Israeli assassination of Hamas leader Ismail Haniyeh in Tehran, the regional militias that the Islamic Republic has armed for decades could play a role in any attack.

Here's a look at Iran's history of arming militias, its allies in the region and what part they could play.

Diplomatic pressure mounted Monday to avoid an escalation between Iran and Israel following high-profile killings that have sent regional tensions soaring, while numerous governments urged their citizens to leave Lebanon.

Israeli Prime Minister Benjamin Netanyahu said late Sunday that his country was "determined to stand against" Iran and its allied armed groups "on all fronts".

U.S. Secretary of State Tony Blinken told his counterparts from the G7 countries on Sunday that an attack by Iran and Hezbollah against Israel could start as early as Monday, three sources briefed on the call told U.S. news portal Axios.

“Blinken convened the conference call to coordinate with close U.S. allies and try to generate last-minute diplomatic pressure on Iran and Hezbollah to minimize their retaliation as much as possible. He stressed that limiting the impact of their strikes is the best chance to prevent all-out war,” Axios said.

Iran said Monday it has the "legal right" to respond to the assassination in Tehran last week of Hamas chief Ismail Haniyeh, an attack blamed on Israel amid the Gaza war.

"No one has the right to doubt Iran's legal right to punish the Zionist regime", Iranian foreign ministry spokesman Nasser Kanani told a regular news conference.

An Israeli airstrike killed an official in the Hamas-run government in Gaza who Israel said was also involved in militant activities.

Hamas confirmed that Abdel-Fattah al-Zariai was killed alongside his mother in an airstrike on Sunday. In a statement of mourning, it identified him as the undersecretary of its Economy Ministry and said he had held other “honorable positions” without referring to any militant role.

The head of Iran’s paramilitary Revolutionary Guard threatened Israel on Monday after the assassination of Hamas leader Ismail Haniyeh in Tehran.

Gen. Hossein Salami, speaking to journalists at an event, warned that Israel was “digging its own grave” with its actions in its war against Hamas in the Gaza Strip, and that it is suspected of carrying out Haniyeh’s killing.

Israel's army said it had not changed "as of now" its policy for protecting civilians, as Iran and Hezbollah are expected to avenge killings blamed on Israel of two senior members.

"I would like to refer tonight to the various reports and rumors that we are on alert for the enemy's response to the territory of the State of Israel," military spokesman Rear Admiral Daniel Hagari said Sunday in an online briefing to journalists.

A missile attack by Yemen's Houthi rebels struck a Liberian-flagged container ship traveling through the Gulf of Aden, authorities said Sunday, the first assault by the group since Israeli airstrikes targeted them.

The Houthis offered no explanation for the two-week pause in their attacks on shipping through the Red Sea corridor, which have seen similar slowdowns since the assaults began in November over Israel's war on Hamas in the Gaza Strip. But the resumption comes after the assassination of Hamas leader Ismail Haniyeh in Iran, the Houthis' main benefactor, amid renewed concerns over the war breaking out into a regional conflict.

Israel is already in a "multi-front war" with Iran and its proxies, Israeli Prime Minister Benjamin Netanyahu told a Cabinet meeting Sunday, as the United States and allies prepared to defend Israel from an expected counterstrike and prevent an even more destructive regional conflict.

Tensions have soared following nearly 10 months of war in Gaza and the killing last week of a senior Hezbollah commander in Lebanon and Hamas' top political leader in Iran. Iran and its allies have blamed Israel and threatened retaliation. Hamas says it has begun discussions on choosing a new leader.

Jordan's foreign minister has met Iran's acting foreign minister in Tehran as the United States and its Arab allies try to prevent a wider war after the back-to-back killings of top Iran-allied militants sparked vows of revenge against Israel.

Ayman Safadi is the first senior Jordanian official to pay an official visit to Iran in over 20 years. Jordan is a close Western ally and helped intercept scores of missiles and drones fired by Iran toward Israel in April. Iran said it was retaliating for the killing of its generals in an Israeli strike in Syria.