In the wake of recent escalations in the Middle East, the U.S. State Department has raised its advisory for Lebanon to level four: "Do Not Travel," up from a level three: "reconsider travel."

"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 U.S. Embassy strongly encourages U.S. citizens who are already in Southern Lebanon, near the borders with Syria, and/or in refugee settlements to depart," the updated advisory reads.

"U.S. citizens in Lebanon should be aware that consular officers from the U.S. Embassy are not always able to travel to assist them. The Department of State considers the threat to U.S. government personnel in Beirut serious enough to require them to live and work under strict security. The internal security policies of the U.S. Embassy may be adjusted at any time and without advance notice," it said.

Even before Tuesday's strike in Beirut, State Department officials have been advising Americans in Lebanon to put together a "crisis plan of action" and "leave before a crisis begins." Assistant Secretary for Consular Affairs Reena Bitter said in a video message earlier this week that those remaining in Lebanon should be prepared to shelter in place "for a long period of time."

While some commercial flights from Lebanon have been disrupted, an official said earlier Wednesday that the State Department assesses that the transportation situation for U.S. citizens is still tenable enough that the department is not considering launching any evacuation efforts at this time, but that those plans are constantly being augmented in case they become necessary.

Level four is the State Department’s most severe classification. Other countries at that rank include North Korea, Afghanistan, Iran, Russia, Ukraine, Syria and South Sudan.

-ABC News' Shannon K. Kingston and Chris Boccia

U.S. Secretary of State Antony Blinken and other officials have held back from publicly speculating whether the assassination of Hamas political leader Ismail Haniyeh will derail cease-fire negotiations, but several U.S. officials familiar with the talks say it certainly isn't good for the prospects of a deal.

The two assassinations of Hamas and Hezbollah leaders in the last 24 hours could have a chilling effect on the mediators, and what appears to be a recommitment to wartime mentality from both Israel and Hamas.

Blinken and others have recently said that an agreement is close, but always with the important caveat that the final gaps are the hardest to bridge.

Israel has also hardened some of its positions in recent weeks and the U.S. has never had full confidence in Hamas leader Yahya Sinwar’s desire to achieve a cease-fire or its insight into his thinking, officials say.

At the State Department briefing Wednesday, its deputy spokesperson, Vedant Patel, declined to speculate whether the assassinations would impact the talks and asserted that the department did not anticipate even a temporary pause in negotiations at this point.

-ABC News' Shannon K. Kingston

Hezbollah has confirmed that military commander Fouad Shukr, also known as Hajj Mohsen, was killed in an Israeli strike on a building in Beirut, calling it a "heinous attack and the major crime."

Hezbollah Secretary General Hassan Nasrallah will speak about the attack at a funeral procession on Thursday, according to Hezbollah.

In his first public statements since Israel killed top Hamas and Hezbollah leaders, Israeli Prime Minister Benjamin Netanyahu warned it would retaliate strongly "against any aggression."

"Challenging days are ahead of us. Since the attack in the Beirut, threats have been heard from all sides. We are ready for any scenario and will stand united and determined against any threat. Israel will charge a price, a very heavy price, for any aggression against us from any arena," Netanyahu said Wednesday.