Syria’s opposition forces have accused the government of President Bashar al-Assad of gassing hundreds of citizens, by one report from the Syrian National Coalition, as many as 1,300, according to Reuters. The attack, if confirmed, would be the world’s worst chemical weapons attack since Saddam Hussein’s gassing of thousands of Iraqi Kurds in 1988, as well as the deadliest attack so far in the Syrian civil war.

The Syrian government denies that it used chemical weapons.

The reports have not been independently confirmed; however, U.N. chemical weapons investigators, who arrived in Damascus three days ago, have been directed to investigate the deaths. Images obtained by Reuters show “scores of bodies including [those] of small children, laid on the floor of a clinic with no visible signs of injuries.”

According to the Associated Press, heavy shelling began around 3 a.m. local time in the capital’s eastern suburbs of Zamalka, Arbeen, and Ein Tarma. Reports from the Syrian Observatory for Human Rights say that the shelling, as well as the sound of fighter jets, was heard by residents of the capital throughout the night and early Wednesday morning.

Observatory director, Rami Abdul-Rahman, citing activists in the area, told the AP that “poisonous gas was fired in rockets as well as from the air in the attack.”

George Sabra, a senior member of the Syrian National Coalition, blamed the al-Assad regime as well as the “weakness of the U.N. and American hesitation” for the civilian deaths. “The silence of our friends is killing us,” he said, according to the AP.

The attack coincides with a visit by a 20-person U.N. chemical weapons team who came to Syria to investigate three sites where alleged attacks have taken place during the last year.

Russia, an ally of the Syrian government, called the reports “alarmist.” Russia has backed President al-Assad’s denials by saying the attacks look like “a rebel ‘provocation’ to discredit him,” Reuters reports.

In June, the White House said it had “high confidence” that the Syrian regime was using chemical weapons against opposition forces. President Obama made the decision to provide more assistance to Syrian opposition forces, including military support, although no action has yet been taken.

Civilian reports to Reuters state that victims looked like they were sleeping, and doctor interviews describe symptoms “they believe point to sarin gas,” a chemical agent that Western governments have accused Syria of having in their chemical weapons stockpile.

A man who had been helping to retrieve victims in the suburb of Erbin told Reuters that when they entered the houses, everything and every person was in place. “They were lying where they had been. They looked like they were asleep. But they were dead,” he stated.

Alisha is a writer and researcher with Ring of Fire. Follow her on Twitter @childoftheearth.