GAZA, May 14 (CNA) - Israel fired artillery and mounted more air strikes on Friday (May 14) against Palestinian militants in the Gaza Strip amid constant rocket fire deep into Israel's commercial centre.

As hostilities entered their fifth day, with no sign of abating, the Israeli military said in a statement shortly after midnight that air and ground forces were attacking the Hamas-run enclave.

"Israeli planes and troops on the ground are carrying out an attack in the Gaza Strip," the Israeli army said in a brief message. The escalation was confirmed by army spokesman John Conricus, although he did not specify the scale of the operation.

Rocket barrages from Gaza swiftly followed.

Although the statement gave no further details, Israeli military affairs correspondents who are briefed regularly by the armed forces said it was not a ground invasion, and that troops were firing artillery from Israel's side of the border.

Residents of northern Gaza, near the Israeli frontier, said they had seen no sign of Israeli ground forces inside the enclave but reported heavy artillery fire and dozens of air strikes.

Israeli Prime Minister Benjamin Netanyahu said on Thursday the campaign "will take more time". Israeli officials said Hamas, Gaza's most powerful militant group, must be dealt a strong deterring blow before any ceasefire.

The sound of explosions echoed across northern and eastern parts of Gaza. Witnesses said many families living in areas near the border quit their homes, some seeking shelter at United Nations-run schools.

Violence 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 109 people were killed in Gaza, including 29 children, over the previous four days, Palestinian medical officials said. On Thursday alone, 52 Palestinians were killed in the enclave, the highest single-day figure since Monday.

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

The UN Security Council will hold a virtual public meeting on Sunday to address the soaring violence, diplomats said on Thursday.

The United States, which had blocked an originally scheduled Friday session and proposed a meeting early next week, agreed to move the session - requested by Tunisia, Norway and China - to Sunday, the same sources said.

The US said earlier on Thursday it wanted to give time for diplomacy.

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