Israel intensified its air strikes on Gaza as rocket barrages hit Israeli towns for a second day in a deepening conflict in which at least 28 people in the Palestinian enclave and two in Israel have been killed.

“The recent rockets in Israel and air strikes in Gaza represent a dangerous escalation of the tensions and violence witnessed over the past days in Jerusalem, including its Old City," Fabrizio Carboni, ICRC regional director for the Middle East, said in a statement. 

"The International Committee of the Red Cross is issuing an urgent call for restraint and de-escalation," he said. 

International humanitarian law prohibits direct and indiscriminate attacks against civilians, any attack must be proportionate, and all necessary precautions must be taken to avoid civilian casualties, Carboni said. 

Hostilities in densely populated areas are bound to result in an "unbearably high human cost" if principles are not upheld, he said in the statement issued minutes before a 13-storey residential tower in Gaza was hit by an Israeli air strike and soon after collapsed, witnesses said. It was not immediately clear if the building had been fully evacuated or if there were casualties.

Carboni called for "rapid, safe and unimpeded movement for ambulances" and for staff and volunteers of both the Palestine Red Crescent Society and the Magen David Adom Society in Israel. 

The Geneva-based ICRC is the guardian of the 1949 Geneva Conventions, which set down international humanitarian law on the protection of civilians and other vulnerable groups such as former combatants.