Hezbollah chief Sayyed Hassan Nasrallah warned Thursday that the group was bound to respond to Israel's killing of its top military commander Fouad Shukur, saying his death and that of Hamas leader Ismail Haniyeh "crossed" red lines.

"You do not know what red lines you crossed," he said, addressing Israel during a speech broadcast at Shukur's funeral.

Israel has communicated a “stern warning” to Hezbollah through Western and regional intermediaries following the recent assassination of top commander Fouad Shukur that any massive attack on Israeli civilians will lead to war, Israeli sources told the Yedioth Ahronoth newspaper.

“Israel intends to target fighters rather than combat-supporting infrastructure and urges Hezbollah to adopt a similar focus,” the sources added.

Britain’s Foreign Secretary David Lammy and Defense Secretary John Healey arrived Thursday in Beirut following a trip to Qatar.

They met with Speaker Nabih Berri and caretaker Foreign Minister Abdallah Bou Habib following their arrival before heading to the Grand Serail for talks with caretaker Prime Minister Najib Mikati.

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.

The Israeli military said late Tuesday that it wanted to avoid any wider war with Hezbollah but that its forces were ready for "any scenario."

"Hezbollah's ongoing aggression and brutal attacks are dragging the people of Lebanon and the entire Middle East into a wider escalation," military spokesman Rear Admiral Daniel Hagari said in a statement.

U.S. Vice President Kamala Harris, the presumptive Democratic election nominee, said Tuesday that Israel had a "right to defend itself" following a strike in Hezbollah's stronghold in Beirut's southern suburbs.

Harris -- who last week struck a tough tone with Israeli Prime Minister Benjamin Netanyahu over Israel's war on Gaza -- also called for a diplomatic solution to reduce the risk of all-out conflict on the Lebanon-Israel border.

U.N. Special Coordinator Jeanine Hennis-Plasschaert on Tuesday said she is "deeply concerned" after Israel's strike in "the densely populated southern suburb of Beirut which resulted in multiple civilian casualties."

"The Special Coordinator underscores once again that there is no such thing as a military solution and calls on both Israel and Lebanon to avail of all diplomatic avenues to pursue a return to the cessation of hostilities and to recommit to the implementation of resolution 1701 (2006)," a statement issued by her office said.

Caretaker Prime Minister Najib Mikati on Tuesday condemned an Israeli strike on Hezbollah's stronghold in Beirut's southern suburbs that killed at least one person and wounded 68 others, days after Israel vowed to retaliate over deadly rocket fire on the occupied Golan Heights that it said came from Lebanon.

Mikati "condemned the blatant Israeli aggression on the southern suburbs of Beirut," the premier's office said in a statement, describing it as a "criminal act" in a "series of aggressive operations killing civilians in clear and explicit violation of international law."