Failure to stop attacks in Syria may lead to more, victims of the sarin gas attack that struck the town of Khan Sheikhoun said, reiterating that chemical weapons have reportedly been used many times during in the country's 11-year war.

Abdel Hamid al-Youssef said 25 members of his family, including his wife and infant twins, were killed when poison gas was dropped on their town in Syria in 2017, in an attack a U.N.-backed inquiry concluded was launched by the Syrian regime.

"In seconds, everything was erased. Life was completely erased," Youssef, 33, said.

The bombardment, in Syria's opposition-held northwest, killed at least 90 people, 30 of them children, Human Rights Watch (HRW), a New York-based rights group, said.

By the time of the strike, Syrian-allies Russia and China had already vetoed efforts at the United Nations to open an investigation by the International Criminal Court (ICC) into war crimes and crimes against humanity in Syria.

As the Khan Sheikhoun attack marks its fifth anniversary, survivors and human rights campaigners say the failure to hold anyone accountable for chemical attacks in Syria could further encourage use of such banned weapons.

The United States and other countries have warned Russia could deploy chemical or biological munitions in its invasion of Ukraine, without providing concrete evidence. The Kremlin has dismissed the statements as "diversion tactics."

"There is no deterrent for Russia," said Youssef, who wants Bashar Assad to be held to account. "Until this day, the criminal is free."

Assad's regime has denied using chemical weapons in the war, which started as an uprising against his rule and has killed at least 350,000 people. Syria signed international conventions outlawing the use of such weapons in 2013.

The details of the Khan Sheikhoun attack are seared into Youssef's memory, starting with the noise of warplanes that launched several airstrikes on the town beginning at 6:30 a.m.

Trying to get his family to safety, Youssef headed toward his parents' home. His wife went ahead as he stopped to aid a neighbor who was was screaming for help.

Youssef said he helped load casualties into a pickup truck. Some were foaming at the mouth.

Youssef lost consciousness as he tried to help his niece. He awoke in hospital hours later, only realizing the scale of the calamity when he returned home that afternoon.

U.S. President Donald Trump's administration responded by firing 59 cruise missiles at the air strip from which it said the attack was launched.

Six months later, a report by an investigative mechanism established by the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) – which enforces treaties banning the use of such arms – said the victims' symptoms were consistent with large-scale poisoning by the nerve agent sarin.

It said it was "confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Shaykhun on April 4, 2017." The town fell to regime forces in 2019.

Five years later, Youssef says he still feels the effects and sometimes faints when he smells strong odors such as household chlorine. The biggest impact, however, has been psychological, he said, adding that he lives in fear.

For survivors of sarin attacks, the effects can include persistent vision problems, gastrointestinal issues and post-traumatic stress disorder, said Professor Alastair Hay, a chemical weapons expert.

"The main impact is usually catastrophic death, and very quickly," he said, adding that more data is needed on the long-term consequences of exposure to chemical weapons.

At the time of the attack, Russia, which threw its military support behind Assad in 2015, said the chemicals belonged to the Syrian opposition, not the regime.

Syria has seen some of the most extensive use of chemical weapons since World War I.

Around 150 cases of alleged uses of chemical weapons in Syria are being investigated by the OPCW and there have been 20 confirmed uses of such weapons, a source familiar with the matter said.

Investigations at the U.N. and by the OPCW special Investigation and Identification Team concluded that Syrian regime forces used sarin and chlorine barrel bombs in attacks between 2015 and 2018.

The deadliest sarin attack of the war to date was in 2013 on opposition-held Ghouta near Damascus that killed hundreds of people but prompted no Western military response. The threat of a U.S. missile strike was averted when Moscow brokered a deal for Syria's chemical weapons to be destroyed by the following year.

Many diplomats and weapons inspectors later concluded Syria's promise to give up its stockpile was a ruse.

Russia and China's U.N. veto of an ICC inquiry on Syria has forced rights advocates to pursue other legal avenues.

Civil society groups have filed complaints over chemical weapons attacks in Syria to judicial authorities in France, Germany and Sweden, where criminal investigations have been opened under laws giving universal jurisdiction for crimes against humanity.

The cases have not yet been brought to prosecution, according to the Open Society Justice Initiative.

Hamid Ketteny, a civil defense rescue worker who says he carried the bodies of six children killed in Khan Sheikhoun, said he helped document the massacre.

Nidal Shikhani, director general of the Chemical Violations Documentation Center of Syria, said he remained optimistic that perpetrators would be held to account, noting the large amounts of evidence gathered by his group and others.

His organization had received requests for evidence from prosecutors on five cases in three European countries, most recently in September.

"These crimes do not fade with the passing of time," he said.