Former President Jimmy Carter criticized the Obama administration Tuesday over its policy against ISIS in the Middle East, telling the Fort Worth Star-Telegram that he believes the U.S. waited too long to act.

"We let the Islamic State build up its money, capability and strength and weapons while it was still in Syria," Carter said. "Then when [ISIS] moved into Iraq, the Sunni Muslims didn't object to their being there and about a third of the territory in Iraq was abandoned."

Carter, the 39th president and former governor of Georgia, a Democrat, said it's difficult for him to figure out exactly what Obama's policy is in the Middle East, noting that, upon leaving office, two of Obama's former secretaries of defense criticized the president's lack of "positive action."

Obama's policy "changes from time to time," Carter said.

According to Carter, the U.S. needs troops on the ground to "direct our missiles and to be sure you have the right target."

U.S. missile strikes have so far reportedly failed to significantly deter the advances of ISIS, and high-ranking members of the al Qaeda offshoot Khorasan were said to have either escaped strike locations along with their high-tech explosive devices, or were not at the strike location in the first place.

"Then you have to have somebody to move in and be willing to fight ISIS after the strikes," continued Carter.

During the interview at a Meadowbrook Habitat for Humanity project, Carter also criticized the Obama administration for the extrajudicial overseas drone killing of four American citizens who were never brought to trial.

"I really object to the killing of people, particularly Americans overseas who haven't been brought to justice and put on trial," he said. "We've killed four Americans overseas with American drones. To me that violates our Constitution and human rights."

One of the Americans killed was 16-year-old Colorado-born Abdulrahman al-Awlaki, who was killed along with his 17-year-old Yemini cousin.