Nearly a dozen of Syria's powerful rebel factions, including one linked to al Qaeda, formally broke with the main opposition group in exile on Wednesday and called for Islamic law in the country, dealing a severe blow to the Western-backed coalition.
The new alliance is a potential turning point, entrenching the schism within the rebellion and giving President Bashar Assad fuel for his long-stated contention that his regime is battling Islamic extremists in the civil war.
The Turkey-based Syrian National Coalition - the political arm of the Free Syrian Army rebel group - has long been accused by those fighting inside Syria of being a puppet promoted by the West and Gulf Arab states supporting the Syrian rebellion.
Wednesday's public rejection of the coalition's authority will likely be extremely damaging for its future in Syria, particularly at a time when the US and Russia are pushing for peace talks.
"If the groups involved stand by this statement, I think this could be a very big deal - especially if it develops into a more-structured alliance instead of just a joint position," said political analyst Aron Lund.
"It basically means that some of the biggest mainstream Islamist forces within the so-called FSA are breaking up with the political leadership appointed for them by the West and Gulf states, to cast their lot with more hard-line and anti-Western Islamists," he said.
The announcement came less than two weeks after the coalition elected an interim prime minister, Ahmad Touma, charging him with organising governance in opposition-held territories that have descended into chaos and infighting.
In a joint statement, 11 rebel groups that are influential in Aleppo province in the north, including Jabhat al-Nusra, which is listed as a terrorist organisation by the US State Department, said they rejected the authority of the Syrian National Coalition as well as Touma's appointment.
A video on the internet showed Abdel-Aziz Salameh, political chief of the Liwaa al-Tawheed brigade that is particularly strong in the northern city of Aleppo, reading the statement.
"These forces call on all military and civilian forces to unite under a clear Islamic framework based on Sharia law, which should be the sole source of legislation," the signatories said.
Ominously, the rebel groups' statement was titled "Communique No. 1", a term used in Arab countries following military coups that suggests the creation of a new leadership body.
It said the rebels do "not recognise" any future government formed outside Syria, insisting that forces fighting inside the country should be represented by "those who suffered and took part in the sacrifices".
The statement highlighted the growing irrelevance of the coalition and its military arm headed by General Salim Idris, who leads the Supreme Military Council supported by the West, amid increasing radicalisation in Syria. The group is seen by many as being out of touch.
Veteran opposition figure Kamal Labwani, a member of the coalition, said the US decision to shelve military intervention in retaliation to the August 21 chemical weapons attack near Damascus and the perceived Western indifference to Syrian suffering was turning fighters in Syria into "monsters".
"We as a coalition are very removed from the ground now. There is no geographic spot we can enter in the liberated areas. The situation is worse than you can ever imagine," he said.
For many rebels, the realisation that even a chemical weapons attack would not trigger military intervention has led to bitterness and more radicalisation. Last week, al Qaeda militants expelled FSA fighters from a town near the Turkish border after some of the worst clashes between the two sides. An al Qaeda commander in the north was assassinated by FSA fighters a day later.
Wednesday's statement came hours after a delegation from the coalition, headed by Ahmad al-Jarba, met US Secretary of State John Kerry in New York. It also came as a team of experts arrived in Damascus to continue investigating allegation on the use of chemical weapons in the civil war.
The US and Russia have been pushing for a peace conference in Geneva. One opposition figure said the rebel announcement breaking from the coalition may be related to concerns it may agree to go to the talks.
"It is part of political jostling for representation ahead of any talks," he said, speaking on condition of anonymity because the he was not authorized to talk about the discussions under way in New York.
Al-Jarba met Wednesday with UN-Arab League envoy Lakhdar Brahimi in New York. Al-Jarba said the coalition expressed readiness to attend talks in Geneva aimed at establishing a transitional government with full executive powers and a clear timetable for an agreement that those in Assad's regime responsible for war crimes against civilians would be held accountable and not be part of a future democratic Syria.
"The time has come to end the conflict in Syria," said al-Jarba, according to a coalition statement. It was not immediately clear if the coalition was relinquishing its previous demand that Assad step down ahead of such talks.
A US official said the United States and its allies were discussing the rebel announcement, adding it's too early to tell what the impact will be. Another US official said the US and its allies are increasingly concerned by infighting between the FSA and al Qaeda militants in northern and eastern Syria.
Both spoke to reporters on condition of anonymity because they were not authorised to publicly discuss Kerry's meetings.
The opposition has long been hobbled by divisions between those in exile and the disparate rebel groups fighting Assad's regime in Syria's civil war, which has killed over 100,000 since March 2011.
The insurgency has increasingly drawn jihadis from all over the world, further adding to the West's reluctance to get militarily involved in the Syrian conflict or send advanced weapons to the rebels. There is growing concern among moderates that the dominant role the extremists are playing is discrediting the rebellion.
Among the signatories of Wednesday's statement are the Islamist-leaning Ahrar al-Sham and Liwaa al-Islam brigades, both powerful rebel factions with large followings on the ground, as well as the al Qaeda-linked Nusra Front. Three of them - the Liwaa al-Tawheed, the Liwaa al-Islam, and the Suqour al-Sham - have until now been part of the Free Syrian Army, considered to be the Coalition's military wing.
Growing rebel infighting may further complicate the work of UN chemical weapons inspectors who face enormous challenges on the ground, including maneuvering between rebel and government-controlled territory.
A team of experts arrived on Wednesday in Damascus to continue investigating what officials from the world organisation have described as "pending credible allegations" of the use of chemical weapons.
The visit of the six-member team, led by Swedish expert Ake Sellstrom, follows a report by the inspectors after a previous trip. The report said the nerve agent sarin was used in an August 21, attack near Damascus.
The US and its allies say Assad's regime was behind the attack that killed hundreds of people. Damascus and its ally, Moscow, blame the rebels for the attack.
The UN experts will be investigating three alleged uses of chemical weapons earlier this year and seeking information on three alleged incidents last month.
UN spokesman Martin Nesirky said the mission will discuss with the Syrian government "information that it may possess regarding allegations it reported on the use of chemical weapons" in incidents on August 22, 24 and 25.
He said the inspectors will visit the village of Khan al-Assal near Aleppo to probe a March 19 incident, as well as two other sites. The inspectors identified them in last week's report as Sheik Maqsood and Saraqueb.
Also Wednesday, activists said Kurdish gunmen captured the village of Hmaid in the northeastern province of Hassakeh after heavy fighting with members of al Qaeda's Islamic State in Iraq and the Levant and Nusra Front. It came nearly an hour after Kurdish gunmen took the nearby village of Dardara.
Omar Mushaweh, a spokesman for Syria's Muslim Brotherhood group which is part of the coalition, blasted the rebel statement and said the infighting is dividing the rebellion at a critical time.
"The only one who benefits from these side wars is the regime," he said.