British PM: Assad one of the "recruiting sergeants" for ISIS

As U.S. President Barack Obama and Russian President Vladimir Putin sharply disagreed over how to handle the crisis in Syria, British Prime Minister David Cameron said on Tuesday that to defeat the Islamic State of Iraq and Syria (ISIS) in the war-torn country, President Bashar al-Assad must go.

"Assad is one of the recruiting sergeants for ISIL because of what he's done to his people. That is one of the reasons why people are flocking to ISIL, to fight for ISIL," Cameron said Tuesday on "CBS This Morning," using an alternative acronym for ISIS.

One of the reasons ISIS has grown, Cameron said, is because of Assad's brutal actions against his own people.

At an address to the United Nations address Monday, Mr. Obama called for a "managed transition" of Assad, saying, "We must recognize that there cannot be, after so much bloodshed, so much carnage, a return to the prewar status quo."

Cameron acknowledged that some observers view ISIS as even worse than Assad and cutting a deal with the Syrian leader to take on ISIS may seem appealing.

"Sounds enticing, but even if you thought it was the right thing to do - which it isn't - it wouldn't work," Cameron said.

As Russia steps up their military support for Assad's regime, Putin made it clear he wants Assad to stay and called for a new alliance with the Syrian president in fighting against ISIS.

"I think Putin understands that Islamist extremist terrorism is against Russia's interests, just as it's against America's interests or Britain's interests," Cameron said. "He knows that's a threat to him. But he has been, up to now, willing to work with Assad. We need to convince him that actually the only way you'll have a Syria free of ISIL is to have a replacement of Assad."

While Cameron said Russians sent their resources because "they felt Assad was on the brink of falling," Putin told CBS News' Charlie Rose on "60 Minutes" that he was trying to protect his country from the potential threat of extremists returning to Russia.

At the end of the day, sending troops into Syria means Russia is invested in the war-torn country's future, Cameron said.

"What we have to do is convince them that it's going to be a pretty poor investment unless there's a transition of the government away from Assad. Because in the end, the Syrian people, those 12 million people who've left their homes, they aren't going to go back to their homes if the butcher is in charge of the country," Cameron said.

The influx of millions of migrants who are fleeing Syria and traveling to Europe has recently overwhelmed European leaders.

While Cameron said Britain will be taking in 20,000 refugees, he defended the U.K.'s support of refugee camps, arguing that settling migrants would encourage them to make a "lethal" trip.

"I think that if you're not careful, you started encouraging people to make that lethal journey," he said. "And we see people getting into these desperately unsafe dinghies, getting out of a Turkish beach and their children and their families dying so we want to stop people from taking that journey.

European countries have been criticized for being slow to step up amid the migrant crisis, but Cameron defended Britain, saying they have been the second most generous donator to Syrian refugee camps after the United States.

"The truth is this: There are 12 million Syrian people have been made homeless by Bashar al-Assad, only 3 percent of them have actually made the journey to Europe. So there's millions left in the region and we should not be encouraging those people to make the journey so we said, 'Put them all into the refugee camps, help them Jordanian government, help the Lebanese government,' and we've done that since day one," Cameron said.

Cameron said "you have to stick to the right path" with the regard to the conflict in Syria and migrant crisis.

"If you ask me is this is the most difficult, intractable problem that President Obama is facing and I face? Absolutely, yes," he said. "We're four years into this. So many people have died. So many people have left their country. But that doesn't mean you give up. Nor should it mean that you go for a sort of phony solution of thinking you could team up with Assad to defeat ISIL, because that would be self-defeating."