ISTANBUL, Turkey - As ISIS has grown, it's used neighboring Turkey, a key U.S. ally, as its staging ground.

For three years, the Turkish government has allowed fighters to stream across its borders into Syria, driven by its desire to topple Syria's dictator, Bashar Al-Assad.

In 2012, CBS News interviewed Mahmoud, a bulldozer salesman from Atlanta, Georgia who returned to his homeland to join the battle against the Syrian regime.

"We come in and out," he said at the time. "The Turkish they are closing eyes, when we cross."

Islamic extremists also took advantage of the Turkish turning a blind eye.

In December, CBS News filmed men crossing illegally into the war zone in broad daylight.

Many militants have been treated in Turkish hospitals, and set up safe houses in Turkish border towns.

For three years, the Turkish government has allowed fighters to stream across its borders into Syria. CBS News

And in a Turkish government refugee camp two years ago CBS News met Syrian men who said they regularly crossed back into Syria to fight, and wanted to establish an Islamic state.

The Turkish government says it's never helped ISIS, and considers it to be a terrorist group.

But Hursit Gunes, a member of Turkey's opposition, claims his government has allowed ISIS to flourish because it prefers the group to the Syrian regime.

He even accuses the Turkish authorities of ignoring the militants' lucrative oil smuggling business on Turkey's border.

"That money could be stopped," said Gunes. "The money they get from smuggling could be stopped if the Turkish government and the neighbor countries had decided that they shouldn't get a coin."

A Turkish government official told CBS News 6,000 foreigners are now banned from Turkey because of fears they could slip across the border to fight with ISIS in Syria or Iraq. But he also said that Turkey has a 500-mile-long border with Syria, and it's simply impossible to stop everyone who wants to join the cause of the Islamic extremists.