The Obama administration has ended its $500 million program to train and equip moderate rebels in Syria, according to media reports.

The program has trained 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.

The program will be replaced with "a far less ambitious program," defense officials said. One part of the program, which had provided small arms, ammunition and vehicles to moderate rebels, will be reduced to providing 5,000 moderates with ammunition only, NBC News reported. The U.S. will also no longer be recruiting moderate rebels, The New York Times reported.

Defense Secretary Ash Carter said Friday during a news conference in London that he "wasn't happy with the early efforts" of the train-and-equip program and that it would be replaced with "a number of different approaches."

"I think you'll be hearing from President Obama very shortly" on the program, Carter said, according to The New York Times.

The original program has endured withering criticism of late. Gen. Lloyd Austin, head of U.S. Central Command, had to appear before the Armed Services Committee in September to answer whether the battle against the Islamic State had reached a "stalemate," and that the U.S. program to train moderate Syrian rebels had failed. Shortly after his testimony, U.S. Central Command revealed that American-trained Syrian fighters gave at least a quarter of their U.S.-provided equipment to an al Qaeda affiliate in Syria.

Last week, four senators penned a letter to the State Department, the Pentagon and the CIA that said the program "goes beyond simply being an inefficient use of taxpayer dollars. As many of us initially warned, it is now aiding the very forces we aim to defeat."

The letter, signed by Sens. Christopher Murphy, D-Conn., Joe Manchin, D-W.Va., Tom Udall, D-N.M., and Mike Lee, R-Utah, called for an end to the program and concluded: "With over 200,000 persons killed, 4 million refugees and 7.6 million internally displaced people, the situation in Syria is absolutely tragic, and we must ensure that any U.S. efforts do not cause additional harm."