UN High Commissioner for Human Rights Louise Arbour said international law stressed the need to protect civilians.
There is an obligation on all parties to respect the “principle of proportionality”, she said.
About 300 Lebanese, most of them civilians, have been killed in the violence. Thirty Israelis, including 15 civilians, have also been killed.
The UN reported on Wednesday that about 100 Palestinians, civilians and fighters, have been killed since the start of the Israeli offensive in Gaza in late June.
Both crises were precipitated by the capture by Hamas and Hezbollah of Israeli soldiers in cross-border operations into Israel.
Along with a massive shelling campaign across Lebanon and Gaza by Israel, Hezbollah has been firing barrages of missiles into northern Israel, targeting urban areas, and Palestinian militants continue their rocket fire into Israel.
“Indiscriminate shelling of cities constitutes a foreseeable and unacceptable targeting of civilians,” Ms Arbour said.
“Similarly, the bombardment of sites with alleged military significance, but resulting invariably in the killing of innocent civilians, is unjustifiable.”
Ms Arbour expressed “grave concern over the continued killing and maiming of civilians in Lebanon, Israel and the occupied Palestinian territory”.
Without pointing to specific individuals, she suggested that leaders could bear personal responsibility.
“I do believe that on the basis of evidence that is available in the public domain there are very serious concerns that the level of civilian casualties, the indiscriminate shelling of cities and so on, on their face raise sufficient questions that I think one must issue a sobering signal to those who are behind these initiatives to examine very closely their personal exposure,” she told the BBC.