Hours after the international organisation that monitors chemical weapons reached a decision on the destruction of Syria's stockpile, the UN Security Council unanimously adopted a similar resolution from the US and Russia.
International inspectors could begin visiting sites within the country later this week.
"This is the first time that the Security Council has proven itself able to act to address the brutal campaign of violence waged by the regime on our people," said the Syrian National Coalition (SNC), a federation of groups which oppose Syrian President Bashar al-Assad.
But they said they were concerned that there are no consequences if Assad does not comply with the deal. They also requested that the Syrian regime be referred to the International Criminal Court for the chemical weapons attack in Ghouta in eastern Damascus last month that human rights groups say killed hundreds of civilians.
The Organisation for the Prohibition of Chemical Weapons (OPCW) agreement, which was voted on by a 41-member council, seeks complete elimination of Syria's weapons by mid-2014, but does not include sanctions if the Syrian regime fails to meet its commitments.
The rebel groups continue to be fractured, although last week 13 groups unified under a single "Islamic Alliance" umbrella. However they also announced that the recognised political opposition groups, SNC and the Syrian Military Council (SMC), do not represent them.
A lack of accountability from the Syrian government on chemical weapons would be an additional obstacle to the political process at Geneva II, a proposed UN peace conference scheduled for mid-November that will aim to resolve the conflict in Syria. In the past two and a half years, more than 110,000 Syrians have been killed, and more than 1mn refugees have fled the country.
The new opposition coalition, which includes some of the country's largest rebel groups, said they would seek an "Islamic framework" for Syria and would fight to implement "the rule of Shariah and make it the sole source of legislation".
This widening gulf between rebel groups is a major blow to American hopes that a peace deal might be brokered in the wake of the recent diplomatic solution to the chemical weapons issue.
Some in Washington see the negotiated agreement on chemical weapons as a diplomatic success, but on the ground it is viewed differently.
"Instead of following through on a threat to at least punish the Assad regime for use of chemical weapons, the entire process has reeked of backdoor dealing with Russia," Aymenn Jaman al-Tamini, a Shillman Ginsburg Fellow at the Middle East Forum told the Media Line.
"The chemical weapons situation has only really helped to alienate members of the Syrian opposition both in exile and on the ground who might have wanted to work with the West on account of the lack of resolve in trying to deal with the chemical weapons attacks."
The Syrian Coalition told the Media Line that their preconditions for attending the Geneva II talks included consequences for the government's apparent use of chemical weapons and a confirmation that the talks were focused on a solution involving a transitional government.
"The coming days will be crucial," said Charles Lister, an analyst at IHS Jane's Counter Terror and Insurgency Centre, "in both demonstrating exactly what form this new Islamic Alliance will take and precisely how the SNC/SMC will react. While the debate within Western political circles regarding the potential provision of military assistance to the SNC/SCM is ongoing, the scope for Western influence over the Syrian opposition has now been diminished considerably."
Notably, the Islamic State of Iraq and Al-Sham (ISIS), the most radical group in Syria, was not included in the announcement. After the large battle in Azaz in northern Syria last week between the Free Syrian Army and ISIS, their exclusion is seen as an attempt to unify opposition groups against the mostly foreign contingent.
One Aleppo local told the Media Line the union was a "good step recognising the danger ahead", going on to explain that ISIS are the first and foremost danger faced in the area.
For others, the announcement was not seen in such positive terms.
"Moderates, secularists, and even moderate Islamists have been fighting an uphill battle to survive in Syria, on the ground and elsewhere," a high-profile Syrian activist, known by the pseudonym "the 47th", told the Media Line. "This last shenanigan just secured the last nail in the coffin of moderate forces."
The Syrian National Coalition has yet to release a statement on the development but the Supreme Military Council's leader, Salim Idriss, has cut short his trip to France to hold urgent talks with the new Islamic Alliance, which includes several groups that were under his authority prior to the announcement.
While chemical weapons inspectors may be on the ground within days, the UN Security Council agreement's absence of threats of force for noncompliance by the Syrian regime will not make Idriss' job any easier as he tries to negotiate a resolution with the rebel groups.