The United States has accused the Syrian government of using chemical weapons against its people.

US State Department spokesman Heather Nauert said on Thursday that reports of chlorine gas being used against civilians in Eastern Ghouta were "very concerning".

"Russia is making the wrong choice by not exercising its unique influence. To allow [the] Syria regime to use chemical weapons against its own people is unconscionable. We will pursue accountability," she said, writing on Twitter.

Russia is Syria's closest ally, and has previously blocked attempts to investigate the suspected use of chemical weapons.

Nauert's comments follow a suspected chlorine gas attack in the rebel-held territory of Eastern Ghouta, on the outskirts of Damascus, on Thursday.

On Saturday, Syria's foreign ministry denied the allegations, according to Reuters news agency.

The White Helmets volunteer rescue group posted photos of the suspected attack on Twitter, under the hashtag "Douma Suffocating".

3 civilians killed and many injured, including suffocation cases among children, women and #CivilDefense volunteers! 28 air raids, 70 surface-to-surface missiles, some loaded w/chlorine gas and 200 artillery shells targeted cities in #Eastern_Ghouta today! #Doumasuffocating pic.twitter.com/yXuyWAcLMe — The White Helmets (@SyriaCivilDef) February 1, 2018

"3 civilians killed and many injured, including suffocation cases among children, women and #CivilDefense volunteers! 28 air raids, 70 surface-to-surface missiles, some loaded w/chlorine gas and 200 artillery shells targeted cities in #Eastern_Ghouta today! #Doumasuffocating", the group wrote on Twitter.

It was unclear whether White Helmets was suggesting the three people who had been killed died of suffocation.

The group claimed that volunteer Bilal Bayram was among the victims who was "suffocated by the chlorine gas attack".

Several children were also affected, the group said.

Rebel stronghold under siege

Eastern Ghouta is the last remaining rebel stronghold near the capital, Damascus, and has been under a government siege since 2013.

It is believed that around 400,000 people still live in the area, which suffers from acute food and medicine shortages.

A ceasefire deal for Eastern Ghouta was recently negotiated in Vienna by the Russians, but when implemented in late January, the truce only lasted a few hours.

"The civil war in Syria has not been at the top of the news agenda for at least weeks, but on Thursday the Trump administration made some serious concerns," said Al Jazeera's Rosalind Jordan, reporting from Washington, DC.

Off camera, reporters have been told that the US government believes the Syrian government still holds chemical weapons, Jordan said.

While the use of chemical and biological agents has been rare since World War I, in recent years the Syrian government and Islamic State of Iraq and the Levant (ISIL, also known as ISIS) have been accused of using such weapons as a tool in the conflict.

Chemical weapons have been used on several occasions during the war, which has claimed at least 400,000 lives and displaced 22 million people since fighting began in 2011.

Last April, autopsy results revealed that chemical weapons were used in an attack which killed more than 80 people in Syria's Idlib province. Syrian President Bashar al-Assad said that suspected attack was a "fabrication" to justify a US military attack.