The United Nations secretary-general on Wednesday warned that deadly Israeli strikes on Lebanon posed a "grave risk" to the fragile US-Iran truce, his spokesperson said in a statement.

"The ongoing military activity in Lebanon poses a grave risk to the ceasefire and the efforts toward a lasting and comprehensive peace in the region. The Secretary-General reiterates his call to all parties to immediately cease hostilities," UN chief Antonio Guterres's spokesman said.

The Lebanese health ministry reported that 182 people were killed and 890 wounded by Israeli strikes on Wednesday, with capital Beirut hit by the most violent bombardment yet since the start of the war between Israel and Hezbollah.

Lebanon was pulled into the war after the Tehran-backed militant group targeted Israel in retaliation for the US-Israel war against Iran.

While Iran and the United States agreed to a two-week ceasefire late on Tuesday, Israel insisted that Lebanon was not part of the truce.

Hezbollah responded by saying it had fired rockets towards Israel, and Iran's parliament speaker Mohammad Bagher Ghalibaf appeared to threaten the ceasefire.

"The Secretary-General strongly condemns the loss of civilian lives and is deeply alarmed by the mounting toll on civilians," the UN spokesman said.

"There is no military solution to the conflict. The Secretary-General continues to call on all sides to avail themselves of diplomatic channels," he added.

The UN rights chief and the International Committee of the Red Cross (ICRC) earlier condemned the "scale of killing and destruction in Lebanon" as "nothing short of horrific."

"Such carnage, within hours of agreeing to a ceasefire with Iran, defies belief," said UN High Commissioner for Human Rights Volker Turk.

French President Emmanuel Macron also joined calls for Lebanon to be included in the ceasefire.