BEIRUT — Chemical weapons inspectors in Syria said Monday that they are being denied access to the site of an alleged chemical attack that was used to justify U.S.-led airstrikes over the weekend, amid growing suspicions that evidence of the incident may have been tampered with.

Pro-government media broadcast interviews with doctors from the area saying that no such attack had occurred and that the victims they treated were suffering from asthma.

A team with the Organization for the Prohibition of Chemical Weapons (OPCW) arrived in Damascus on Saturday at the invitation of the government to investigate the alleged chemical attack, which prompted the strikes against three Syrian chemical weapons facilities early Saturday local time.

Two days later, the fact-finding team said it has still not been granted permission by Syrian authorities to visit Douma, the town in the Eastern Ghouta suburbs of Damascus where residents and monitoring groups said the attack took place.

The U.S. ambassador to the OPCW, Kenneth Ward, said he suspects that Russians may have tampered with the evidence. Russia, a longtime ally of the Syrian government, intervened militarily in 2015 to help turn the tide of the civil war.

“It is our understanding the Russians may have visited the attack site. We are concerned they may have tampered with it with the intent of thwarting the efforts of the OPCW fact-finding mission to conduct an effective investigation,” Ward said in comments at a closed-door meeting of the OPCW in The Hague that were later made public.

“Unfettered access essential. Russia & Syria must cooperate,” the account of the British delegation to the OPCW tweeted, expressing concern that access to Douma was being denied.

According to OPCW Director General Ahmet Uzumcu, Syrian and Russian officials have cited “security issues” for the refusal to allow the team to visit the town. Instead, he said, the team members have been told they can interview 22 witnesses who will be brought to Damascus by the authorities.

The suspicions of tampering heightened concerns that the truth about the suspected April 7 chemical attack may never be known. The rebels who had controlled Douma for six years surrendered the day after the alleged incident, which left dozens of apparently uninjured men, women and children dead in an apartment building with foam coming out of their mouths, according to video footage.

Russian and Syrian troops have since deployed in the area, which the Syrian army declared fully under government control on Sunday. Videos broadcast by Russian and Syrian media last week showed Russian troops visiting the building where the footage was filmed, and Russian Foreign Minister Sergei Lavrov told reporters that they had found no evidence of a chemical attack there.

Russia told the OPCW meeting in The Hague that it has found proof that the evidence of a chemical attack was staged by the United States and Britain.

“Not a single witness or affected patient in hospital has been found. Nor any traces of chemical ammunition have been identified. Instead, we managed to find those who participated in filming the faked video, which was eventually presented as ‘proof’ of the chemical attack,” said Alexander Shulgin, Russia’s representative to the OPCW, according to the Tass news agency.

Witnesses, survivors and medical workers have told journalists that scores of people were treated for breathing difficulties that night and that the victims emitted a strong smell of chlorine, which has been widely used in the past as a weapon of war.

In interviews broadcast by the pro-government al-Ekhbariya television channel, 13 medical workers, including nine doctors, said the symptoms they treated suggested that the patients were suffering from asthma, not the effects of a poison gas. A nurse told the interviewers that airstrikes on the night of the alleged chemical attack had ignited fires that triggered asthma symptoms in many people. As medical workers treated those who were suffering asthma attacks, someone shouted “chemicals, chemicals,” prompting staff to hose down patients with water as a precaution, the nurse said, explaining the filmed scenes of what appeared to be the aftermath of a chemical attack.

Activists from the area, some of whom have relocated to rebel-held areas of northern Syria under the terms of the rebel surrender deal, said the medical workers had been coerced into making the statements in return for being allowed to remain in their homes.

Two activists, one of whom joined the evacuation to the north and another who remained in Douma, said government officials went into the area’s hospitals and clinics last week, identified workers who were present on the night of the attack, and took them to Damascus to make what they said were forced statements.

They included some of the medical staff seen in the TV interview denying the attacks had taken place, according to two of the activists. “They had no option,” said one, who is still living in Douma.

Michael Birnbaum in Brussels and Louisa Loveluck in Beirut contributed to this report.