David Cameron has called for an "unconditional, immediate, humanitarian ceasefire" in Gaza to bring a halt to more than three weeks of violence which has claimed the lives of about 1,100 Palestinians and 55 Israelis.

In what was reported to be the heaviest night of bombardments since the current conflict began, a fuel container at the Gaza Strip's only power plant was hit by Israeli tank shells, while at least two explosions hit a media centre housing local TV and radio as well as a number of Arab satellite news channels.

Israeli aircraft and tank attacks on symbols of Hamas control in the tiny Palestinian enclave followed a warning in a televised speech by Prime Minister Benjamin Netanyahu of a "prolonged" campaign in Gaza.

Speaking during a visit to Slough, Berkshire, Mr Cameron said: "What it's time for is unconditional, immediate, humanitarian ceasefire.

"What we're seeing is absolutely heartbreaking in terms of the loss of life, and the pictures that everyone has seen on their television screen are really heart-rending and everyone wants to see this stopped, so an immediate unconditional ceasefire, that is what is required."

The Prime Minister made clear that he blames Hamas, the political movement which controls Gaza, for the outbreak of the current rounding of fighting, saying: "Hamas must stop attacking Israel with rocket attacks. That is how this started. It's completely unjustified and they need to stop as part of the ceasefire."

But a former senior British diplomat called for talks with Hamas to bring it into the Middle East political process.

Sir John Holmes was UN emergency relief co-ordinator during the last Israeli invasion of Gaza in 2009, and said that five years on the current situation was "very much the same".

"I fear that, unless we escape this infernal logic of violence on both sides, in five years' time we will be in exactly the same situation again," Sir John told BBC Radio 4's Today.

"So something has to change - obviously the fighting has to end very quickly because the bloodshed is completely unacceptable from every point of view, but the politics at the same time.

"Hamas is not going to go away and I think that we have accept that, instead of just isolating Hamas, we need to talk to them."

Sir John, who is co-chair of the International Rescue Committee UK, added : " I accept Israel is going to say 'Well, that is just rewarding terrorism' but that is the kind of language and attitude we have to get away from.

"The rest of the world has to say 'We can't go on like this'. We have to talk to Hamas... Let's talk to them as we did talk to others and try to bring them into the political process and stop pretending they are going to go away."