The UN's envoy to the Middle East Robert Serry on Tuesday condemned Israel's killing of civilians in Gaza.

On Tuesday afternoon, an Israeli tank fired at a family playing football outside their home in the eastern part of the Ash-Shaja’iya neighborhood in Gaza City.

The Israeli military said it regretted harm to innocent civilians, but blamed Hamas, who it said operated in civilian areas.

In a statement issued by his spokesman, Serry condemned the attack and expressed concern at the escalating situation in Gaza and southern Israel.

"Serry condemns the killing of three Palestinian children and their uncle and the wounding of thirteen other civilians by an Israeli tank shell in the Gaza Strip earlier today," the statement said.

"Israeli firing into densely populated areas is extremely dangerous and appalling incidents such as today's raise serious questions."

The Special Coordinator also condemned the sharp escalation in rocket fire from Gaza, which he said endangered Israeli civilians.

Gaza militants have launched a barrage of projectiles at Israeli forces operating in the coastal enclave and across the border.

An Israeli military spokeswoman said two Israeli civilians were lightly injured by mortar fire from Gaza on Saturday.

Serry said the UN was working urgently to prevent any further escalation, and called for "an urgent halt to all acts of violence and full respect of international humanitarian law by all parties before more civilian lives are lost."