The United States said it was working "around the clock" to avert an all-out war in the Middle East, as Israel remained on high alert Tuesday for potential Iranian retaliation for two high-profile killings.
US President Joe Biden, whose country has sent extra warships and fighter jets to the region in support of Israel, held crisis talks on Monday with his national security team.
Biden and his top diplomat Antony Blinken sought to calm tensions that have soared since a suspected Israeli attack in Tehran last Wednesday killed Ismail Haniyeh, the political leader of Palestinian militant group Hamas.
Biden called King Abdullah II of Jordan, which helped down Iranian drones and missiles in an attack on Israel in April, while Blinken called top officials in Qatar and Egypt, the key intermediaries seeking a ceasefire in the 10-month Israel-Hamas war in Gaza.
"We are engaged in intense diplomacy, pretty much around the clock, with a very simple message -- all parties must refrain from escalation," Blinken said after joining other top officials in a White House meeting.
Iranian President Masoud Pezeshkian hit out on Monday at what he called the "criminal acts" of Israel "against the oppressed and defenceless people of Gaza" as well as Haniyeh's killing.
"The Islamic Republic of Iran is in no way seeking to expand the scope of war and crisis in the region, but this regime will definitely receive the response for its crimes and arrogance," Pezeshkian said during talks with a senior visiting Russian official, according to the official news agency IRNA.
The killing of Haniyeh -- which Israel has not directly commented on -- came hours after an Israeli strike on Beirut killed the military chief of Lebanon's Hezbollah movement, Fuad Shukr.
Israel held Shukr responsible for a rocket attack in the annexed Golan Heights that killed 12 children, calling him the "right-hand man" of Hezbollah chief Hassan Nasrallah.
Nasrallah was due to give a speech Tuesday to mark one week since Shukr's death.
His Iran-backed group has engaged in near-daily cross-border clashes with Israeli troops since the day after Hamas attacked Israel in early October.
On Tuesday, four Hezbollah fighters were killed in an Israel strike on southern Lebanon, according to a security source.
A European diplomat in Tel Aviv said "a coordinated response" from Iran and its proxies was expected but de-escalation efforts persisted.
"That doesn't mean there will be a simultaneous response from all fronts," he added, declining to be identified as he was not authorised to speak on the issue.
"We're telling them they have to stop playing with fire, because the risk of flare-ups is higher than at any time since October 7," he said.
Turkey on Monday joined multiple governments calling on their citizens to leave Lebanon, where Hezbollah is based, while China urged increased caution.
Numerous airlines have suspended flights to Lebanon or limited them to daylight hours.
Lebanese national carrier Middle East Airlines put on extra flights for people wanting to leave or return, a company source said.
As tensions rose, the Jeddah-based Organization of Islamic Cooperation is to meet on Wednesday at the request of "Palestine and Iran", to discuss developments in the region, an OIC official said.
Iraqi sources said a base hosting US troops in Iraq came under rocket fire on Monday, after an American strike on July 30 killed four pro-Iran Iraqi fighters.
The United Nations' rights chief Volker Turk called on "all parties, along with those states with influence, to act urgently to de-escalate what has become a very precarious situation".
Italian Foreign Minister Antonio Tajani and his Iraqi counterpart Fuad Hussein in a joint statement Monday "agreed to make every effort to avoid a regional escalation". Italy holds the rotating presidency of the G7 group of countries.
The Israel-Hamas war in the Gaza Strip, triggered by the Palestinian group's October 7 attack on Israel, has already drawn in Iran-backed militants in Syria, Lebanon, Iraq and Yemen.
The Hamas attack resulted in the deaths of 1,197 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Palestinian militants seized 251 hostages, 111 of whom are still held captive in Gaza, including 39 the Israeli military says are dead.
In Tel Aviv on Monday thousands of Israelis gathered to mark the fifth birthday of child hostage Ariel Bibas, and to call for the liberation of him and his family.
Israel's retaliatory military campaign in Gaza has killed at least 39,623 people, according to the Hamas-run territory's health ministry, which does not give details of civilian and militant deaths.
In the occupied West Bank, Palestinian officials said that Israeli forces killed eight people in two separate raids on Tuesday.
