(CNN) - Chemical weapons experts who have been waiting for five days in Syria to inspect the site of a suspected gas attack were again delayed Wednesday, after gunfire at a UN security team forced it to turn back.
A fact-finding team from the Organisation for the Prohibition of Chemical Weapons (OPCW) was expected to enter the city of Douma on Wednesday to determine whether banned substances were used in a strike there 11 days ago. UK officials say around 75 people were killed in the attack.
The team has faced several obstacles in getting to Douma, raising concerns that any chemicals that may have been used could disintegrate by the time experts get there, or that evidence could be manipulated.
The OPCW said Wednesday that it was not sure when its team could be deployed to Douma, considering the hostile security conditions the UN team experienced.
At a first site, the UN team withdrew after a large crowd gathered, OPCW director general Ahmet Üzümcü said in statement. "At Site 2, the team came under small arms fire and an explosive was detonated. The reconnaissance team returned to Damascus."
A UN source told CNN that no one from the UN team was injured.
The United States, the United Kingdom and France have blamed Syrian President Bashar al-Assad's regime for the attack and together carried out airstrikes over the weekend in response, with the support of other Western nations.
US officials have said they believe chlorine and sarin gas were dropped there. Both Syria and its most powerful ally, Russia, deny a chemical attack took place.
The UK's envoy to the OPCW said on Monday that Syria and its most powerful ally, Russia, had blocked access to the experts, which both countries denied.
Confusion has also swirled in recent days over why the team has failed to gain safe access to Douma.
Syrian state TV and the White Helmets volunteer rescue group said separately on Tuesday that the OPCW team had arrived in the area, only to be contradicted by a US State Department spokeswoman.
'Every day matters'
Jerry Smith, a former chemical weapons inspector in Syria and founder of the RameHead Consulting International security firm, warned that a substance like chlorine could "disappear in a couple of hours."
"Every day matters," he told CNN recently. "The greater the time between when it's released and when it's detected, the chances of it being found reduces."
Nerve agents, however, can often be detected for years.
The US envoy to the OPCW, Kenneth Ward, said on Monday he was concerned Russia may have "tampered with" the site of the Douma attack.
"This raises serious questions about the ability of the (team) to do its job," Ward said.
Russia has openly announced that its representatives have been to the site, and a CBS journalist has also accessed the site, reporting from the location in recent days.
Russian Foreign Minister Sergey Lavrov told the BBC's "HARDtalk" that he guaranteed there had been no Russian tampering, adding his country had sent experts to the site and found "no trace" of chemical weapons.
Russian state-run media outlet TASS said a team from its conflict monitoring center in Syria had examined Douma on April 9, two days after the alleged attack, and found no sign of chemical use.
Russia and US lock horns
Already-strained relations between Russia and the Western allies that launched the coordinated strikes on Syria have deteriorated in recent days. Information on the attack site is murky, and different sides are giving conflicting details and accusing one another of cooking up conspiracy theories.
Russia has claimed the April 7 attack was "faked," or even "staged" with the help of British intelligence agencies. Britain denies the allegation.
The Russian military said Tuesday in state media it had discovered a chemical laboratory and warehouse in Douma allegedly run by "militants." CNN could not corroborate that allegation.
US and Russian representatives once again went head to head Tuesday over Syria at a UN Security Council meeting.
Russian Ambassador to the UN Vassily Nebenzia called the behavior of the US, the UK and France "hypocritical."
"Not even a day passed after the missile strikes when their organizers started putting forward strange and political initiatives," Nebenzia said.
Nebenzia went on to call Washington and its allies "self-appointed executioners" and accused them of pressuring Russia to change its position after the latest round of airstrikes and a promised new round of sanctions.
Russian media reported that the Security Council meeting, convened at the request of Russia, focused on the humanitarian situation in ISIS' former stronghold of Raqqa and displaced persons in the Al-Rukban refugee camp on the border with Jordan
Kelley Currie, a US representative to the United Nations, accused Russia of distracting from the atrocities committed by the Assad regime.
"In order to do that, Russia has asked this council to focus its attention on the one part of Syria where the Assad regime isn't pummeling civilians to death with barrel bombs or banned chemical weapons," Currie said.
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