"The number of people who have crossed into Syria from Lebanon fleeing Israeli airstrikes -- Lebanese and Syrian nationals -- has reached 100,000," Filippo Grandi said on X.

"The outflow continues," he warned.

He said his UN refugee agency (UNHCR) was "present at four crossing points, alongside local authorities and (the Syrian Red Crescent) to support new arrivals".

The mass displacement into war-torn Syria began a week ago, on September 23, the UNHCR told AFP on Saturday.

Israel has widened its attacks in recent days to include Lebanon as well as the Gaza Strip, targeting Iran's regional ally, Hezbollah.

Strikes on Hezbollah targets in Lebanon killed it leader, Hassan Nasrallah, on Friday.

In the last week, Israeli bombardment has killed more than 700 people in Lebanon, including 14 paramedics over a two-day period, according to Lebanon's health ministry.

By Friday, 30,000 people had crossed into Syria, according to the UNHCR.

Its representative in Syria, Gonzalo Vargas Llosa, said around 80 percent were Syrian nationals and 20 percent Lebanese.

"Most are women and children, although some males have crossed. About half are children and adolescents," he told reporters.

He stressed the fleeing people were "arriving in a country that has been suffering from its own crisis and violence for more than 13 years, as well as from economic collapse".

"People fleeing the bombing arrive in Syria exhausted, traumatised and in desperate need of help."