As many as 100 people – including about 25 children – are dead in Syria after an apparent chemical attack on rebel-held areas, according to a relief group, an act that might be considered a “war crime” said a US official.

The Tuesday airstrikes in north-western Syria used a toxic gas that left many people choking and vomiting, and some with foam coming out of their mouths, Rami Abdel Rahman of the Britain-based Syrian Observatory for Human Rights told dpa.

Some activists posted pictures online of dead children piled on top of each other. The attack occurred in the area of Khan Sheikhoun, in Idlib province.

Accusations quickly flew. The Syrian National Coalition, an opposition group, accused regime planes of carrying out the attack, and said they used a gas similar to sarin.

Syria’s military denied the accusation in a statement.

The army “denies using any toxic or chemical agents in Khan Sheikhoun today, and it did not and never will use it anywhere,” read the statement.

Meanwhile, US President Donald Trump called the attack “reprehensible,” and blamed the Syrian government, but noted that the Damascus would not feel able to launch such attacks had former US president Barack Obama punished them for similar attacks in the past.

At the same time, Trump’s secretary of state, Rex Tillerson, said countries like Iran and Russia, which have supported Syrian President Bashar al-Assad, should acknowledge his “brutal, unabashed barbarism.”

The latest death toll data came from the Union of Medical Care and Relief Organization (UOSSM). It also counted 400 wounded and noted that its own medical staff “were affected by the attack, and rushed to the ICU, while treating patients.”

A US official stressed the incident was still under investigation, but “if it is what it looks like, it is clearly a war crime.”

A few hours after the first attack, planes launched fresh airstrikes targeting medical facilities in Khan Sheikhoun.

The raids “came hours after a massacre was committed by fighter jets in the city,” Abdel Rahman added.

“Most of the people inside the town have moved to the outskirts because raids are still targeting heavily,” Abu Majd, an activist in the city, told dpa by phone.

The first raids took place near a bakery near the northern entrance of Khan Sheikhoun, Majd said.

The Observatory said Turkey opened the Bab al-Hawa crossing on the Syrian-Turkish border and began allowing the wounded in.

The Observatory added that planes also targeted the northern Idlib towns of Jisr al-Shoghor and Salqeen, closer to the borders with Turkey, killing 18 people.

The Syrian National Coalition described the attack as a “massacre” and called for a United Nations Security Council emergency meeting, echoing calls from France and Britain. A meeting is now set for Wednesday. UN agencies had also begun investigating the reports.

Bashar al-Assad’s regime has been accused of using chemical weapons several times since the civil war began in 2011, including in an attack on the area of Ghouta, near Damascus, with the chemical agent sarin. More than 1,400 died in the 2013 incident.

In 2014, the Organization for the Prohibition of Chemical Weapons (OPCW) said Syria handed over its declared stockpile of chemical weapons.

In March, UN investigators said Syrian security forces attacked civilians with chlorine gas in Aleppo late last year, one day after China and Russia vetoed new sanctions against Syria for its use of chemical weapons.

On Tuesday, the UN’s chemical weapons agency expressed serious concern about the attack and said the OPCW’s Fact Finding Mission “is in the process of gathering and analysing information from all available sources.”

Tillerson was blunt in his assessment of Syria’s role.

“Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable.”

Tillerson called for a “genuine ceasefire” in Syria and for Russian and Iran to exercise influence over al-Assad to prevent such attacks in the future.

“As the self-proclaimed guarantors to the ceasefire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths,” he says.

But Trump said the US carried its share of the blame.

“These heinous actions by the Bashar al-Assad regime are a consequence of the past administration’s weakness and irresolution. President Obama said in 2012 that he would establish a “red line” against the use of chemical weapons and then did nothing.”