Britain's foreign minister David Lammy on Wednesday called for a transparent investigation into an Israeli air strike on a UN building in Gaza.

"Appalled a UN compound in Gaza was hit this morning," Lammy wrote on X. "This incident must be investigated transparently and those responsible held to account.

Meanwhile, Israel announced renewed ground operations in Gaza on Wednesday and issued what it called a "last warning" to residents of the Palestinian territory to return hostages and remove Hamas from power.

Israeli forces this week conducted the deadliest wave of air strikes since the start of a truce in January, killing hundreds of people, according to the health ministry in the Gaza Strip.

The military said it had "begun targeted ground operations in the central and southern Gaza Strip to expand the security perimeter and create a partial buffer between the north and south".

As Israel kept up its renewed bombardment despite a chorus of calls from foreign governments to preserve the ceasefire, long lines of fleeing civilians filled the roads of Gaza on Wednesday.

Families with young children fled northern Gaza for areas further south, fearing for their lives after Israel urged civilians to leave areas it described as "combat zones".

Fred Oola, senior medical officer at the Red Cross field hospital in Rafah, said the renewed strikes had shattered the relative calm of the past two months.

"Now, we can feel the panic in the air... and we can see the pain and devastation in the faces of those we are helping," he said in a statement.

Addressing the "residents of Gaza", Israeli Defence Minister Israel Katz said in a video statement: "This is the last warning."

Separately, The Gaza civil defence agency said an Israeli strike on Wednesday killed 14 members of the same family in the northern town of Beit Lahia.

Agency spokesman Mahmud Bassal told AFP that "at least 14 martyrs and several wounded from one family" were transferred to a hospital "after the (Israeli) occupation carried out an air strike targeting a house where a mourning tent was held", adding several people were feared buried under the rubble.

The casualties and victims had gathered at the home of a family who were mourning the recent loss of their loved ones when the strike occurred. They were then transferred to the Indonesian Hospital, according to the Palestinian News Agency. (AFP)