William Hague has led fresh calls for a ceasefire in the Middle East after the bloodiest day of the Gaza crisis increased growing concerns over the safety of innocent civilians caught up in the conflict.

The Foreign Secretary warned Israel that it risked losing international sympathy if it escalated its campaign against Palestinian militants into a ground invasion. But he repeated his insistence that Hamas bore "principal responsibility" for the violence and urged its leaders to cease a barrage of missiles still being fired at Tel Aviv and other areas.

Israeli prime minister Benjamin Netanyahu told his cabinet the Israel Defence Forces were prepared for a "serious broadening" of the operation - amid fears it will send in troops.

Israeli airstrikes continued to hit the enclave during the early hours of Monday morning as the violence entered its sixth day. Health officials said rockets hit two houses belonging to a single family, killing two children and a woman and injuring 30 others - half of them children.

It came less than 24 hours hours after several women and children were among 11 reportedly killed in an airstrike on a residential area as Israel expanded its targets to what it said were the homes of wanted militants. Two media centres were also attacked - with Hamas communications the alleged target - that wounded six local journalists and damaged international broadcasters' equipment.

The World Health Organisation (WHO) said it is deeply concerned about the situation and its impact on the health and lives of civilians in the area, while charity Save the Children said hundreds of thousands of youngsters are trapped in houses in Gaza facing prolonged power cuts and depleting supplies of food and water.

Medical officials said 37 civilians are among 77 killed so far. Three Israeli citizens have also died.

United Nations Secretary General Ban Ki-moon added his voice to growing calls for a ceasefire saying he was heading to the region to personally appeal for an end to the violence. "This must stop," he said. "Any further escalation will inevitably increase the suffering of the affected civilian populations and must be avoided."

Speaking to Sky News, Mr Hague said the UK was "gravely" concerned about this situation. "We call on Hamas again to stop the rocket attacks on Israel, it is Hamas that bears principal responsibility for starting all of this and we would like to see an agreed ceasefire - an essential component of which is an end to those rocket attacks," he said.

"The Prime Minister and I have both stressed to our Israeli counterparts that a ground invasion of Gaza would lose Israel a lot of the international support and sympathy they have in this situation. A ground invasion is much more difficult for the international community to sympathise with or support, including the United Kingdom."