United Nations investigators on Thursday said that Syria’s ruling regime under Bashar al-Assad was responsible for the deadly sarin gas attack on the rebel-held town of Khan Sheikhun in April, AFP reported. More than 87 people were killed in the attack in Syria’s Northwest province of Idlib on April 4.

“The panel is confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhun on 4 April 2017,” said the report, which AFP accessed. While Western intelligence reports had earlier blamed the regime for the attack, Syria and its ally Russia said a rebel weapon may have detonated on the ground.

The UN report has increased global pressure on Assad’s regime. The US, which has been gaining ground against the Islamic State group in several territories, has renewed calls for Assad to step down.

“The reign of the Assad family is coming to an end, and the only issue is how should that be brought about,” said US Secretary of State Rex Tillerson. British Foreign Secretary Boris Johnson urged the “international community to unite to hold Assad’s regime accountable”.

The report was compiled by the UN and the Organization for the Prohibition of Chemical Weapons’ Joint Investigative Mechanism, which was created by the UN Security Council in 2015, Reuters reported. Its mandate was renewed in 2016 for a year, and is due to expire in mid-November.

Earlier this week, Russia had vetoed a proposal to extend the organisation’s mandate. The Joint Investigation Mechanism had previously blamed the Syrian government forces for three chlorine gas attacks in 2014 and 2015.