Mr Bush also praised Benjamin Netanyahu for taking a tough stance against Hamas.
Former US President George W Bush has slammed the Hamas group for last week's terror attack on Israel and said members of the Gaza Strip-based outfit are "cold-blooded killers".
"You're dealing with cold-blooded killers... You can make all kinds of excuses why they are, but they are," Mr Bush said in an interview while speaking with US-based historian Mark Updegrove on Friday while extending support to Israel in the country's ongoing war that has killed more than 3,000 people after violence escalated with Hamas last week on Saturday.
The former US President also warned that this war is "going to be ugly for a whole" and more tough days are ahead for Gaza.
His statement comes as Israeli troops started their ground offensive on Gaza to wipe out Hamas from the territory, warning Palestinians to leave their homes within 24 hours.
Calling the assault on Israel an "unprovoked attack", Mr Bush said Hamas "terrorists are willing to kill innocent people to achieve an objective."
Speaking at the event, Mr Bush further said the US must stand with Israel and for this, there should be "no ands, ifs or buts".
"I am pretty confident the US will stand squarely with Israel," the former US President said.
Mr Bush also praised Benjamin Netanyahu for taking a tough stance against Hamas and said "negotiating with killers is not an option" for the Israeli Prime Minister.
"His job is to ... protect his country. And anyway, we'll find out what he's made out of," Mr Bush said, referring to Mr Netanyahu.
On Iran's alleged link to the Hamas attacks in Israel, the 77-year-old former Republican stated said, "I don't know. I don't get the intelligence anymore. Their stated objective is the destruction of Israel. That's what the leaders say. And in this world, you got to take what they say seriously. And Iran has been pretty good at using surrogate terrorist groups, Hezbollah being the key word of course."