The destruction of bridges and homes on Sunday marks a significant escalation in Israel's military campaign in Lebanon, which was pulled into the regional war on March 2 when armed group Hezbollah fired into Israeli territory.
International law generally prohibits militaries from attacking civilian infrastructure, and the United Nations human rights chief has criticised Israel's actions in Lebanon, particularly its use of widespread evacuation orders.
Sunday's strike pulverised a crossing on Lebanon's coastal highway that ran through farmland and was one of the main routes linking southern and central Lebanon.
An Israeli military spokesperson had announced the army would strike the bridge earlier on Sunday.
Earlier, an Israeli was killed in his car near the border with Lebanon after what the military described as a "launch" from Lebanese territory. It was the first Israeli civilian death linked to fire from Lebanon in the current war. Two Israeli soldiers have also been killed in fighting in southern Lebanon.
Israel's strikes on Lebanon have killed more than 1000 people, including nearly 120 children, 80 women and 40 medical personnel, according to Lebanon's health ministry. Lebanese authorities do not otherwise distinguish between civilians and militants.
Defense Minister Israel Katz said on Sunday that the military had been ordered to destroy all bridges over the Litani River used for "terrorist activity," to prevent Hezbollah militants and weapons from moving south.
The Israeli military had already destroyed three bridges in southern Lebanon in the last 10 days.
Katz also said the military was ordered to accelerate the demolition of Lebanese homes in "frontline villages" to neutralise threats to Israeli communities.
He described the approach as similar to the model used in Beit Hanoun and Rafah in Gaza, where the military created buffer zones by clearing and demolishing buildings near the border.
Ramzi Kaiss, Lebanon researcher at Human Rights Watch, told Reuters that international law requires armed actors to take into account the civilian harm caused by strikes on infrastructure like bridges, even if the targets were being used for military purposes.
"If all these bridges are struck, and the region that is south of the Litani becomes isolated from the rest of the country, then the civilian harm is going to be so immense that you have a humanitarian catastrophe as people still living in the south won't be able to access food, medicine and other basic needs," Kaiss said.
Destroying homes in southern Lebanon wholesale would amount to wanton destruction, which is a war crime, he added.
The Israeli military says its troops are carrying out what it describes as ground manoeuvres and targeted raids on Hezbollah militants and weapons stores in southern Lebanon. Israeli officials say the air and ground campaigns are aimed at protecting residents in northern Israel near the Lebanese border from Hezbollah attacks.
