"We cannot see in Lebanon what we are seeing in Gaza. And we cannot allow what has been happening in Gaza to continue," Guterres said at a news conference at the UN headquarters.
The UN chief said tens of thousands of people in northern Israel and southern Lebanon have been displaced by the fighting and humanitarian access in Lebanon continues to be constrained.
"Stop playing with fire across the Blue Line, de-escalate, and bring hostilities to an end in accordance with Security Council Resolution 1701," he said.
Tensions have escalated along Lebanon’s border with Israel since the Israeli army launched a deadly military offensive on the Gaza Strip following Hamas’ Oct. 7 attack.
An exchange of cross-border fire has recently been taking place between Hezbollah and Israeli forces, in the deadliest clashes since the two sides had fought a full-scale war in 2006.
The UN chief also reiterated his call for a cease-fire in Gaza.
"The onslaught on Gaza by Israeli forces over these 100 days has unleashed wholesale destruction and levels of civilian killings at a rate that is unprecedented during my years as Secretary-General," he said.
Stressing that the vast majority of those killed are women and children, Guterres said: "Nothing can justify the collective punishment of the Palestinian people."
Humanitarian aid
Guterres also outlined certain basics for an effective humanitarian operation in Gaza.
Guterres said the UN and its partners cannot effectively deliver aid while Gaza is under such heavy and unrelenting bombardment by Israel.
He also noted that the aid operation faces "significant hurdles" at the Gaza border and major impediments to distribution within Gaza.
"This includes repeated denials of access to the north, where hundreds of thousands of people remain. Since the start of the year, just 7 of 29 missions to deliver aid to the north have been able to proceed," he added.