One year after the deadly chemical attack on rebel-held areas outside Damascus, the victims and their families have yet to see those behind the mass killings held responsible, a human rights group said Thursday.

In the early hours of Aug. 21, 2013, rockets carrying the nerve agent sarin slammed into the rebel-held suburbs of the Syrian capital of eastern Ghouta and Moadamiyeh. Hundreds were killed in what the U.N. chief called the "worst use of weapons of mass destruction in the 21st century."

The attack is almost certainly the single deadliest event in Syria's civil war -- a conflict that has killed more than 170,000 people since it began in March 2011.

Following the chemical assault, U.N. inspectors conducted a swift investigation that determined rockets loaded with sarin had been fired from an area where the Syrian military has bases. But the U.N. probe's limited mandate did not authorize the experts to identify who was responsible for the attack.

The Syrian opposition and its allies, including the United States, accused Damascus of carrying out the attack. President Bashar Assad's government denied responsibility, and blamed the rebels.

The Obama administration threatened to carry out punitive airstrikes against the Syrian government, touching off a flurry of diplomacy that eventually resulted in Assad accepting a U.S.-Russia brokered deal to relinquish his chemical arsenal.

Over the past 11 months, a joint mission by the U.N. and the Organization for the Prohibition of Chemical Weapons has overseen the removal of all of Syria's declared chemical stockpile of 1,430 tons from the country. More than 80 percent of those materials, which include mustard gas and precursors to sarin, have been destroyed so far, according to the OPCW.

Nadim Houry, deputy Middle East and North Africa director at the New York-based Human Rights Watch, welcomed the removal of Syria's chemical arsenal, but said its destruction "will do nothing for the hundreds of victims who died a year ago and the relatives who survive them."

"Closure of the chemical weapons issue in Syria will be possible only when those who ordered and executed the Ghouta attacks have been held to account and are behind bars," Houry said in a statement Thursday.

Questions also linger over whether Assad is hiding undeclared poison gases or attacking rebels with chlorine. While not specified as a chemical weapon, chlorine is a toxic industrial gas. The use of any such material as a weapon is illegal under international law.

In May, an OPCW fact-finding mission found evidence that chlorine gas was used in fighting between rebels and Assad's government. The OPCW stopped short of saying which side was to blame.

The main Western-backed Syrian opposition group used Thursday's anniversary to urge the international community to fully follow through with the mission to destroy Syria's chemical program.

"Much work remains to be done as questions over discrepancies in the regime's initial declaration of chemical substances remain unanswered," the Syrian National Coalition said in a statement.

"Also, the remaining chemical weapons production facilities must be completely destroyed and the regime's ability to produce, stockpile, and use chemical weapons neutralized," it said.

It also mourned the loss of those killed a year ago in the attack outside Damascus, saying the "families of the victims deserve closure."

"The international community must send a strong message to Assad, and indeed all dictators, that terrorizing their populations will not pass unchecked, and that it is just a matter of time before those responsible for war crimes and crimes against humanity are brought to justice," the Coalition said.