The United Nations has said there is 'no justification' for attacks against civilians, as cross-border violence mounts in Gaza.

It follows the targeted killing of a Palestinian Islamic Jihad leader inside Gaza on Tuesday.

The UN Special Coordinator for the Middle East Peace Process, Nickolay Mladenov, expressed growing concern over increasing rocket fire across the border between Israel and the Gaza Strip.

He said: "I am very concerned about the ongoing and serious escalation between Palestinian Islamic Jihad and Israel".

According to local reports, the attack not only killed the militant leader but also his wife and others.

Meanwhile, the Health Ministry in Gaza enclave reported on Wednesday that retaliatory airstrikes by Israel had killed 13 Palestinians - including three children.

This brings the death toll since Tuesday to 23.

Mr Mladenov added: "The indiscriminate launching of rockets and mortars against population centres is absolutely unacceptable and must stop immediately.

"There can be no justification for any attacks against civilians."

Noting that the continuing escalation is "very dangerous", he said it is "yet another attempt to undermine the efforts to improve the dire socio-economic conditions in Gaza" and "another devastating conflict" must be prevented.

"The UN is working to urgently de-escalate the situation", he said.

Israel Defense Forces (IDF) reported that more than 100 rockets had been fired into Israel during the two days of fighting.

News reports said around 50 people had been treated for injuries.

Palestinian Islamic Jihad reportedly confirmed that those killed in Gaza included members of its military wing.