A United Nations security team was fired upon in the Syrian town of Douma this week while carrying out reconnaissance at the site of a suspected chemical attack, reported Reuters. This will further delay the arrival of chemical weapons inspectors at the site.

Organisation for the Prohibition of Chemical Weapons Director General Ahmet Üzümcü said a security team of the United Nations Department of Safety and Security had decided to carry out a reconnaissance before the arrival of the inspectors. They were forced to withdraw after the gunfire, reported The Financial Times. He was speaking at a meeting at the organisation’s headquarters at The Hague on Wednesday.

“On arrival at site one, a large crowd gathered and the advice provided by the United Nations Department of Safety and Security was that the reconnaissance team should withdraw,” he said. “At site two, the team came under small arms fire and an explosive was detonated.”

The source of the gunfire is unclear, reported The Guardian. The Syrian government said on Sunday it had “purified” Douma and the broader area of eastern Ghouta, which had been under siege for years and subjected to a number of chemical attacks, of “terrorists”.

US Defence Secretary Jim Mattis blamed the Syrian government for delaying the inspectors’ arrival. He claimed the government has a history of trying to “clean up the evidence before the investigation team gets in”, according to Reuters.

Syrian ambassador to the UN Bashar Ja’afari denied the claim. “I have informed you before that the Syrian government has implemented all necessary measures to facilitate the arrival of the fact finding mission to Syria”, according to The Financial Times.

The inspectors of the Organisation for the Prohibition of Chemical Weapons arrived in Damascus on Saturday and were bound for Douma on Tuesday, according to reports. They were to start their work at the two sites on Wednesday.

Western powers have blamed the suspected chemical attack on the Syrian government, which has denied any involvement. The attack, which is reported to have killed 42 people, led the United States, Britain and France to fire missiles at Syrian targets on Saturday in retaliation.