Senior US lawmakers on Wednesday blasted defense officials over America's strategy against the Islamic State group, and vented outrage over allegations military officials "cooked" intelligence to overstate coalition successes in Syria.

Members of the Senate Armed Services Committee were astonished to learn a half-billion-dollar program to train moderate Syrian rebels had gotten off to a disastrous start, and said it was time to implement "safe zones" over parts of Syria to prevent forces loyal to President Bashar al-Assad from dropping barrel bombs.

The hearing came as President Barack Obama's administration draws criticism over US-led efforts to "degrade and ultimately destroy" the Islamic State group, which despite dozens of daily air strikes maintains control over parts of Iraq and Syria and has affiliates across the region from Lebanon to Yemen, Libya and Egypt.

"We are seeing the latest manifestation of this failed policy, the flood of people pouring out of the Middle East that has led to the worst refugee crisis in Europe since World War II," said committee chairman Senator John McCain, who in 2008 ran against Obama as the Republican candidate for president.

- 'Train and equip' problems -

Central to the hearing was the issue of local forces fighting IS jihadists. Unwilling to commit US ground troops in the region, the Obama administration in January launched a "train and equip" program for Syrian opposition fighters as part of a broader push to work with locals there and in Iraq.

The original target was for around 5,400 vetted Syrians each year for three years. But the program faltered, with many would-be fighters failing the screening process.

In July, an Al-Qaeda affiliate attacked an initial graduating group of 54 troops -- and the Pentagon has not said what happened to them all.

When pushed on how many of that group remained in the fight, General Lloyd Austin conceded it was only a handful.

"It's a small number," Austin said. "The ones that are in the fight is... we're talking four or five."

Austin heads the US military's Central Command, better known as CENTCOM, which is overseeing efforts against the IS group. He was joined by Under Secretary of Defense for Policy, Christine Wormuth.

Austin acknowledged the program had gotten off to a "slow start" but said it was a mere "complement to all the other things that we're doing."

Wormuth said more than 100 additional fighters were in the "pipeline."

"This number is much smaller than we hoped for at this point, partly because we put our volunteers through a very vigorous screening process," she said.

Republican Senator Kelly Ayotte said the number was a "joke."

The situation looked better in Iraq, with 13,000 Iraqi soldiers already trained at multiple sites and more than 3,000 currently undergoing training, Wormuth said. Nonetheless, IS fighters still control several important Iraqi cities including Fallujah and Ramadi.

- 'Cooked' intelligence? -

With the Pentagon acknowledging the fight against IS is likely to take years, lawmakers turned their attention to allegations that senior military officials altered information to downplay the strength of Islamic State and Al-Qaeda's branch in Syria. The scandal eruped following a Daily Beast report, that said more 50 US intelligence analysts had complained of intelligence manipulation.

Austin said the Pentagon's inspector general was looking at the allegations.

"Based upon the findings, you can be assured that I will take appropriate actions," he said.

Republican Senator Tom Cotton said he was surprised to hear allegations of intelligence being "cooked," given that the existing reports "are not painting a very pretty picture how this campaign is going."

Lawmakers asked Austin to ensure whistleblowers in the case were protected.

- No-fly zones -

With no end in sight for Syria's bloody civil war, which has killed more than 240,000 people and displaced millions since 2011, politicians called for a stepped-up US response.

McCain has long supported no-fly zones to prevent Assad bombing civilian populations, and he reiterated those calls Wednesday.

"We need to put an end to Assad's ability to use airpower against his people, especially his use of horrific barrel bombs, which is one of the leading killers of innocent Syrians," McCain told the committee.

Democratic Senator Angus King said it was time to rethink a strategy about a "safe zone" -- or a no-fly zone.

"We have allowed this ... atrocity to go on too long," King said. "It's impacting us. It's impacting the rest of Europe. I really think that there should be a rethinking of the nonintervention strategy."