Robert Ford, former U.S. ambassador to Syria, said he resigned his post because of the Obama administration's failed Syria policy and he predicted that another 300,000 to 400,000 Syrians will die if nothing is done to stop the bloodshed.

Asked by Sharyl Attkisson on "Full Measure" Sunday why he had resigned, Ford said: "The reason is, I could no longer defend the administration's policy on Syria."

He then repeated three times that he knew the policy "wasn't working."

He resigned just after administration officials announced that Obama had approved a plan to provide training and equipment to moderate Syrian rebels. The $500 million program was ended this week after it came under fire for training only a handful of rebels in the fight against the Islamic State. Many of the weapons the U.S. sent to the moderates quickly passed to al Qaeda in Syria and other jihadist groups.

Ford told Attkisson that the worst refugee crisis since World War II would only grow worse and up to 400,000 will die if the crisis in Syria is allowed to fester.

Ford's comments come on the heels of several setbacks to the administration's Syria policy. Russia announced it was launching airstrikes inside Syria to wipe out the Islamic State there, but the U.S. and its allies charge that the Kremlin is bombing moderate Syrian opposition. Putin is allied with Syrian President Bashar Assad, who is accused of using chemical weapons against his own people.

Attkisson asked if the U.S. is handling the refugee crisis properly and what is causing it, as well as whether we "should be" handling the crisis at all.

Calling the crisis "a terrible human tragedy," Ford responded that trying to fix the refugee problem in Europe will not solve the "refugee problem at all because it goes back to the horrible fighting and destruction in Syrian cities, and now well over 200,000 Syrians killed."

"And, the number is rising very quickly," said Ford. "It will be, I hate to say it, but it will be 300,000 and if it goes on, 400,000."

Over 4 million Syrians have fled the country, now in its fifth year of civil war. Many European countries have been unable to handle the tide of refugees, most of whom say they would like to return to a peaceful Syria.

The former ambassador said that Syria needs "a new national unity government" that can put an end to extremism and begin "to rebuild the country with international support."

"That's the way you're going to being not only to stop the flow of refugees going out, but actually then to encourage many Syrians to come home," said Ford. "Rebuilding Syria will take decades."

Ford spent his career in the foreign service. He served as the U.S. ambassador to Algeria from 2006-2008 and ambassador to Syria from 2010-2014.

Considered "one of the best Arabists" during his tenure at the State Department, Ford hasn't been shy criticizing Obama's Syria policy in the past. In an interview with CNN shortly after his resignation, Ford said he could not "defend the American policy" because it had been "unable to address either the root causes of the conflict in terms of the fighting on the ground" or the "growing extremism threat" from the Islamic State.

Ford said there "really is nothing" that's been successful in our policy there "except the removal of about 93 percent of some of Assad's chemical materials."

"But now [Assad's] using chlorine gas against his opponents in contravention of the Syrian government's agreement in 2013 to abide by the Chemical Weapons Convention," said Ford shortly after his resignation. "The regime simply has no credibility, and our policy is not addressing the Syrian crisis as it needs to, frankly speaking."