Israel's military and Hezbollah have accused each other of violating the terms of a ceasefire that came into affect on Wednesday, local time.
On Thursday morning, the Israeli military said its air force struck a facility it claimed was being used by Hezbollah to store missiles.
The military said it also opened fire on Thursday towards what it called "suspects" with vehicles arriving at several areas in the southern zone, and accused Hezbollah of breaching the ceasefire.
Hezbollah politician Hassan Fadlallah accused Israel of violating the deal.
"The Israeli enemy is attacking those returning to the border villages," he said.
"There are violations today by Israel, even in this form."
Earlier, Israeli tank fire hit five towns and some agricultural fields in southern Lebanon, state media and Lebanese security sources said, saying at least two people were wounded.
The ceasefire was scheduled to take effect from 4am local time on Wednesday.
It involved a 60-day transition as Israeli troops and Hezbollah soldiers both leave the area north of the Israeli border and south of the Litani River, followed by a permanent end to hostilities — including ground clashes, air attacks and the rebuilding of Hezbollah infrastructure.
The agreement held for its second day before the alleged breach.
Hezbollah MP Hassan Fadlallah accused Israel of attacking people returning to their villages in south Lebanon.
"The Israeli enemy is attacking those returning to the border villages," Mr Fadlallah told reporters after a parliament session.
"There are violations today by Israel, even in this form."
Lebanese families displaced from their homes near the southern border have tried to return to check on their properties. But Israeli troops remain stationed within Lebanese territory in towns along the border.
Israeli Prime Minister Benjamin Netanyahu said he had instructed the military not to allow residents back to villages near the border.
Lebanon's speaker of parliament Nabih Berri, the top interlocutor for Lebanon in negotiating the deal, had said on Wednesday that residents could return home.
Israeli strikes kill at least 21 in central Gaza, medics say
On Thursday, the World Health Organization (WHO) warned of a "catastrophic" shortage of medicines, food, shelter and fuel in war-torn Gaza.
Despite the ceasefire in Lebanon, there has been no let-up in fighting between Hamas and Israel.
WHO chief Tedros Adhanom Ghebreyesus said since the war between the two sides began on October 7 last year, 90 per cent of people in Gaza had been displaced from their homes and were now living in tents.
"This leaves them vulnerable to respiratory and other diseases, (while) cold weather, rain and flooding are expected to exacerbate food insecurity and malnutrition," he said.
Israeli military strikes killed at least 21 Palestinians across the Gaza Strip on Thursday, medics said, as forces stepped up their bombardment of central areas and tanks pushed deeper into the north and south of the enclave.
Six people were killed in two separate air strikes on a house and near the hospital of Kamal Adwan in Beit Lahiya in the northern Gaza Strip, while four others were killed when an Israeli strike hit a motorcycle in Khan Younis in the south.
In Nuseirat — one of Gaza's eight historic refugee camps — Israeli planes carried out several air strikes, destroying a multi-storey building and hitting roads outside mosques.
At least 11 people were killed in those strikes, health officials said.
Medics said at least two people, a woman and a child, were killed in tank shelling that hit western areas of Nuseirat, while an air strike killed five others in a house nearby.
Residents of Rafah, near the border with Egypt, said tanks pushed deeper into the north-west area of the city.
There has been no Israeli comment on the latest fighting.
Gaza ceasefire negotiations on hold
Israel's 13-month campaign in Gaza has killed nearly 44,200 people and displaced nearly all the enclave's population at least once, Gaza officials say. Vast swathes of the territory are in ruins.
It was sparked by an attack on October 7, 2023, by Hamas on Israel, which killed around 1,200 people saw more than 250 hostages captured, Israel has said.
Months of efforts to negotiate a ceasefire in Gaza have yielded scant progress, and negotiations are now on hold.
"I hope a ceasefire will happen like it did in Lebanon … I just want to take my children to see my land, my house, to see what they did to us, I want to live in safety," said Amal Abu Hmeid, a displaced woman in Gaza.
"God willing we will have a truce," she said.
She told Reuters life was "beautiful" before the war, but now "there is nothing beautiful, it's all gone".
"Our houses are gone, our brothers are gone, and no one is left. Now we hardly get … one meal a day. We can't even get bread."
Israeli Foreign Minister Gideon Saar said on Thursday that Israel would finish the war in Gaza when it "achieves its objectives" of returning hostages being held by Hamas in Gaza and ensuring the group no longer controls the strip.
Mr Saar said Israel did not intend to control civilian life in Gaza and that he believed peace was "inevitable" but could not be based on "illusions".
He also decried the ICC's decision to issue arrest warrants for Israeli Prime Minister Benjamin Netanyahu and his former defence minister as having "no justification".
Mr Saar said Israel had appealed the decision and that he believed the United States would punish the court for it.
Reuters/ABC
