Image copyright Getty Images

Once again the outspoken US Vice President Joe Biden has been forced to apologise for blunt remarks, this time accusing America's Sunni allies in the Middle East of supporting militant extremists in Syria.

The issue was not so much in what he said - Obama administration officials have long been making similar complaints - but that he said it publicly just as the US has secured regional support for a coalition against Islamic State (IS) militants.

His comments also exposed very different views between the US and Syria's neighbours about who's to blame for the rise of IS.

"Our biggest problem was our allies," Mr Biden told students at the Harvard Kennedy School.

"The Turks… the Saudis, the Emirates, etc, what were they doing? They were so determined to take down (Syrian President Bashar al) Assad and essentially have a proxy Sunni-Shia war, what did they do? They poured hundreds of millions of dollars and tens, thousands of tonnes of weapons into anyone who would fight against Assad."

These policies ended up helping militants linked to al-Qaeda and ultimately IS, he said.

Mr Biden also claimed that Turkey admitted it had let too many foreign fighters cross its border into Syria.

The incandescent response from Ankara and expressions of "astonishment" from the United Arab Emirates led Mr Biden to "clarify" that he didn't mean the allies had intentionally facilitated the growth of IS or other violent extremists.

Image copyright AP Image caption IS's ranks have been bolstered by foreign extremists crossing Syria's borders

But there is little doubt about the flow of weapons, money and fighters from these countries into Syria.

US officials and regional analysts have long described direct, largely unregulated, funding of extremist groups by sympathetic individuals in the Gulf states - the Treasury Department recently sanctioned what it said were three such Kuwait-based financiers.

At the official level, Gulf countries led by Saudi Arabia openly declared support for arming the rebels early on in the conflict, and some of them backed Islamist factions.

Qatar and Turkey provided support to the Syrian Muslim Brotherhood and other Islamist militias and "some of what they sent got to (al-Qaeda linked) Jabhat al-Nusra", according to a former administration official.

What those allies were seeking was greater US leadership Robert Ford, Former US ambassador to Syria Biden says sorry to UAE over SyriaWhat is Islamic State?

Qatar's foreign minister Dr Khalid Bin Mohammad Al-Attiya went so far as to tell a 2012 forum at the International Institute for Security Studies that he was "very much against excluding anyone at this stage, or bracketing them as al-Qaeda".

Unlike the other rebel groups, Islamic State has its own sources of revenue, including until recently a booming business in oil smuggling into Turkey. US officials have accused Ankara of, at best, turning a blind eye to the black market trade. Pressing the government to clamp down on it was a key focus of a recent visit by the Secretary of State John Kerry.

And despite denials by the Turkish president Recep Tayyip Erdogan, the steady flow of fighters, including foreign extremists, across Turkey's long and porous border with Syria is well-documented.

Western journalists are among the many witnesses who've seen hundreds of people cross the border illegally. They've met foreign fighters in Syria who've come through Turkey, visited base houses for foreign jihadis on the Turkish side of the border, and interviewed those who've smuggled European recruits into Syria.

Image copyright Getty Images Image caption Recep Tayyip Erdogan (left) has denied turning a blind eye to IS smuggling

Christopher Harmer, a senior naval analyst at the Institute for the Study of War, told the BBC that IS fighters were allowed to use Turkey "at a minimum as a free movement zone", but Ankara provided more direct "logistical support" to al-Nusra rebels at times during the civil war.

Ironically, Joe Biden made his candid comments at a time when all of this is changing.

Alarmed by Islamic State's rapid advance into Iraq and its seizure of territory, the Gulf countries are beginning to close down funding streams to extremist groups. Turkey has taken steps to police the border and crack down on oil smuggling.

But their view of various threat levels continues to differ from that of the US - they, like Syria's non-Islamist rebels, see al-Nusra fighters as valuable allies mainly focused on the battle against the Assad regime - as does their reading of the initial stages of the conflict.

"What Biden said about the allies aggravating the problem of extremism is true," says Robert Ford, the former US ambassador to Syria and now a resident scholar at the Middle East Institute. "But at same time what those allies were seeking was greater US leadership."

Image copyright Getty Images Image caption The flood of refugees from IS-controlled areas has led to a humanitarian crisis

They claim that the United States' failure to take decisive action earlier facilitated the rise of Islamic State by giving extremists the upper hand.

"It's an open contest as to who made the most mistakes," says Anthony Cordesman, an analyst at the Centre for Strategic and International Studies in Washington. "There was a window of time during which, if the US had supported moderate rebels and put pressure on Assad, we might have been able to avoid these problems. Many people who were developing options for the administration feel that is the case."

Officials in Ankara say Mr Biden's apology has closed the issue, and a recent parliamentary vote authorising the use of force in Syria sets the stage for a more active Turkish role in the anti-IS coalition.

But senior Turkish officials have made clear that support will remain qualified, given Turkey's complex dynamics as a front-line state and its reservations about the consequences of the limited American intervention.

As for Washington's other Sunni allies, the Gulf states and Jordan, they appear relieved that Washington has finally made the decision to use the military force for which they've long been calling.

"I think their governments view both Assad and ISIS to be problems, so they're happy to have the Americans go in (to Syria) part way," said the former administration official.

"I dare say they're hoping now that Obama's crossed this part of the Rubicon, he'll go all the way."