Palestinian militants fired more rockets into Israel's commercial heartland on Thursday as Israel kept up a punishing bombing campaign in Gaza and massed tanks and troops on the enclave's border.

Axar.az reports that the four days of cross-border fighting showed no sign of abating and Israeli Prime Minister Benjamin Netanyahu said the campaign "will take more time".

Violence has also spread to mixed communities of Jews and Arabs in Israel, a new front in the long conflict. Synagogues were attacked and fighting broke out on the streets of some towns, prompting Israel's president to warn of civil war.

At least 87 people have been killed in Gaza, including 18 children, over the past four days, Palestinian medical officials said. Hospitals already under heavy pressure because of the COVID-19 pandemic have faced further strain.

Seven people have been killed in Israel: a soldier patrolling the Gaza border, five Israeli civilians, including two children, and an Indian worker, Israeli authorities said.

Worried that the region's worst hostilities in years could spiral out of control, the United States is sending an envoy, Hady Amr. Truce efforts by Egypt, Qatar and the United Nations have so far offered no sign of progress.

U.S. President Joe Biden called on Thursday for a de-escalation of the violence, saying he wants to see a significant reduction in rocket attacks.

Militants fired rocket salvoes at Tel Aviv and surrounding towns, Israel's commercial heartland, with the Iron Dome anti-missile system intercepting many of them. Communities near the Gaza border and the southern desert city of Beersheba were also targeted.