With more than two million people living in Gaza, which is 25 miles wide, what would Israel consider before such an attack and how could Hamas respond?

Here, we look at some of the issues surrounding such a potential operation.

What is the possibility of a ground offensive - and why hasn't it happened yet?

Analysts say such operations take time to plan.

Mr Clarke said: "As and when the Israelis move in, and I'm certain they will move in, it will be a multi-pronged attack - I'm also pretty sure about that."

But he added time is needed to plan out the operation and to call people up for armed service.

Israel's bombing campaign is designed to go after the ammunition supplies and command facilities of Hamas.

There will then be no alternative but for the Israeli Defence Forces to invade the whole Gaza Strip, Mr Clarke added.

When could an Israeli invasion of Gaza begin?

"I think it will be hours or days," said Mr Clarke.

"The Israelis won't want to delay on this.

"They may be holding back because they want their unity government in place to get the political top cover before they move further.

"But they'll want to do this as quickly as they possibly can.

"I think this will be a matter of a day or two days.

"I'll be amazed if this time next week, we were still talking about an air campaign that didn't have this ground operation."

How will the offensive be launched?

The geography of Gaza "matters" and will play a major role in how any attack unfolds in the early stages.

Mr Clarke said he is also "fairly certain" any offensive will be "seaborne as well as airborne".

He highlighted there is more room for tanks to make advances from the northern and southern sides, instead of from the "middle" due to the "big ridge".

Mr Clarke explained: "I think it would be from the land side and from the seaward side, probably.

"Geography matters here because in northern Gaza, that's slightly more open country - and in Khan Younis (southern Gaza), that's slightly more open country - in the middle there's a big ridge, of course, in between Gaza and present Israel."

What could be used to launch the offensive and what are the challenges?

The Israeli military would face difficulties depending on the areas they are targeting.

Mr Clarke said: "In the north and the south, you could use tanks and armoured vehicles, but in the centre - west of the ridge - it would be very difficult because the further into Gaza you go, the more heavily populated it is and tanks and armoured vehicles are pretty vulnerable in that respect.

"And, so, it will be an infantry sort of operation - it's got to be groups of infantry, obviously, with armoured vehicles backing them up.

"But they've got to, as it were, winkle out any fighters.

"If Hamas are going to make a fight of this - they may or may not - they can defend every building and it will become an infantry-heavy operation in which the armour will have to sort of hang back and then do the later job that will be required."

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How would Hamas have been preparing for an Israeli ground assault?

Mr Clarke said the fact some areas have already been "flattened" by the Israeli military will make a ground offensive "far more" of a challenge.

"Hamas must have known this was going to happen," he said.

"They will have prepared. They work a lot in tunnels and bunkers. They've really extended their tunnel network.

"Their intelligence is better than it used to be. Their weapons are better than they used to be.

"It's also possible they'll use the tunnels to get out and just live to fight another day, and then infiltrate back in, in a month or two months' time.

"But if they decide that they're going to fight this, then they can work from the tunnels and the bunkers and they can make it very, very difficult for the Israelis.

"And every piece of rubble is a defensive bunker for somebody."

Will there be any consideration for the Israeli hostages captured by Hamas?

The welfare of the hostages during any ground operation is a "worry" for Israel, said Mr Clarke.

Its military will try to "conduct the offensive in such a way as to rescue as many hostages as possible".

"I'm sure they can't rescue them all, but they will try their best," he added.