At least 30,000 displaced people have sought protection in shelters in Lebanon since hostilities between Israel and Hezbollah escalated this week, the United Nations refugee agency said on Tuesday, while many more were expected to join them.

The Israeli army has conducted air strikes across Lebanon since Monday, after Hezbollah fired rockets at Israel late on Sunday, ​in ⁠reaction to U.S. and Israeli strikes against Iran.

"Conservative estimates suggest that nearly 30,000 people were hosted and registered at collective shelters," said UNHCR spokesperson Babar Baloch.

"Many more slept in their cars on the side of roads or were still stuck in traffic jams," he added.

The number of displaced people will grow much higher, and so far 21 shelters were opened by the Lebanese government, the U.N. World Food Programme said.

There has also been an increase in the number of Syrian refugees moving from Lebanon back into Syria, the UNHCR said, adding that it was putting a contingency plan in place in case of possible further influx.

Lebanon has the highest concentration of refugees per capita in the world, hosting about 1.5 million Syrians among a population of about 4 million Lebanese.

More than 6 million Syrians had fled as refugees after conflict broke out in Syria in 2011, with most heading to Turkey, Lebanon and Jordan.

Children in Lebanon's residential areas are being put in immediate danger by Israeli air strikes, and seven children have been killed and 38 injured since Monday, the U.N. children's agency UNICEF said.

"Each new escalation expands the circle of harm. Residential areas, schools and critical infrastructure are being affected," UNICEF spokesperson Ricardo Pires said.

-Reporting by Emma Farge in Geneva; Additional reporting by Olivia Le PoidevinEditing by Ludwig Burger and Andrei Khalip/Reuters