Switzerland has urged its nationals to leave Lebanon, noting the highly volatile situation in the country.

Hezbollah confirmed Wednesday that Israel killed a top commander after a rare strike in Beirut.

U.N. Secretary-General Antonio Guterres has denounced strikes on Beirut and Tehran as a "dangerous escalation," after Israel targeted a top Hezbollah commander in Lebanon and Hamas said its political chief was killed in Iran.

"The scretary-general believes that the attacks we have seen in South Beirut and Tehran represent a dangerous escalation at a moment in which all efforts should instead be leading to a ceasefire in Gaza" and "the release of all Israeli hostages," spokesman Stephane Dujarric said in a statement.

The United States has raised its travel advisory on Lebanon to its highest "do not travel" classification, citing rising tensions between Israel and Hezbollah after a strike on southern Beirut killed a top commander of the armed group.

"Do Not Travel to Lebanon due to rising tensions between Hezbollah and Israel. If you are in Lebanon, be prepared to shelter in place should the situation deteriorate," the advisory said Wednesday, adding that citizens in southern Lebanon or in refugee settlements should leave.

Hezbollah has mourned its top commander Fouad Shukur, whose body was recovered from the rubble of an Israeli strike in south Beirut, as fears mounted of a wider conflict.

Shukur will be buried on Thursday and Hezbollah leader Sayyed Hassan Nasrallah will speak at his funeral to outline the Iran-backed movement's position, the group said in a statement.

Two back-to-back strikes in Beirut and Tehran, both attributed to Israel and targeting high-ranking figures in Hamas and Hezbollah, have left Hezbollah and Iran in a quandary.

Analysts agree that both strikes hit too close to home to pass without a response, and were serious security breaches for Iran and Hezbollah. Calibrating that response to restore deterrence without sparking an even more damaging escalation may be the most delicate balancing act in nearly a year of teetering on the brink of a regional war.

Israeli Foreign Minister Israel Katz has sent a letter to dozens of foreign ministers, calling on them to demand “an immediate cessation of Hezbollah’s attacks, its withdrawal to north of the Litani River, and its disarmament in accordance with U.N. Security Council Resolution 1701.”

“Israel is not interested in an all-out war,” he continues, “but the only way to prevent it is the immediate implementation of Resolution 1701.”

Caretaker Prime Minister Najib Mikati on Wednesday described Israel’s deadly strike on Haret Hreik as “a strike on initiatives, pacification efforts and understandings.”

“We will continue to work to rescue our country and protect our society from any danger,” Mikati added, during an emergency cabinet session tackling the strike on Beirut’s southern suburbs and its repercussions.

U.S. Defense Secretary Lloyd Austin has spoken with Israeli Defense Minister Yoav Gallant regarding the “threat” posed by Hezbollah, the Pentagon said, hours after Israel said it killed Hezbollah’s most senior military commander Fouad Shukur in a rare strike on Beirut’s southern suburbs.

“They discussed the threats to Israel posed by a range of Iranian-backed terrorist groups, including Lebanese Hezbollah,” the readout said without referring to the strike on Shukur and the killing of Hamas leader Ismail Haniyeh in Tehran, an operation also blamed on Israel.

Anger simmered after Israel struck a southern Beirut suburb that is also a Hezbollah stronghold, with young men chanting in support of the group at the site of the strike.

Israel hit Hezbollah's stronghold in retaliation for rocket fire from Lebanon that killed 12 children over the weekend in the occupied Golan Heights, saying it had targeted the commander responsible for the attack.

Hezbollah condemned Israel's killing of Hamas leader Ismail Haniyeh on Wednesday, warning his death will only increase the resolve of Iran-backed "resistance" groups.

Hamas said Haniyeh was killed in an Israeli strike in the Iranian capital, where he was attending the swearing-in of the country's new president, and vowed the act "will not go unanswered".