Syrian flags flew in Douma on Monday, security forces stood on street corners and Russian military police patrolled the streets.
United Nations spokesman Stephane Dujarric said Wednesday the measures are needed in view of the small arms fire and explosive device detonated during a visit by the United Nations advance team Tuesday.
He did not specify who might have opened fire on the United Nations reconnaissance mission on Tuesday, nor did any other official.
The United States accused Russian Federation on Monday of blocking worldwide inspectors from reaching the site of the poison gas attack. Syrian officials have reportedly provided the investigators with a list of 22 people from the region that they can interview, raising further concerns about Syria's efforts to control the narrative surrounding the attack.
Syrian anti-aircraft defences shot down missiles fired at the Syrian air base of Shayrat in Homs province late on Monday and another base northeast of the capital, Damascus, Syria's state television and pro-Iranian Hezbollah media said.
The US and France say they have evidence poison gas was used in the April 7 attack in Douma, killing at least 40 people, and that Syrian President Bashar Assad's military was behind it.
More than 70 people died in the April 7 attack - which ultimately drew a military response from the United States, Britain and France.
Israel has long said Iran was expanding its influence in a belt of territory that stretches from the Iraqi border to the Lebanese border, where Israel says Iran supplies Hezbollah with arms.More news: Eagles host Falcons in season opener, travel to London in Week 8
For example, chlorine, which the United States assesses was used in the April 7 attack, is a common industrial chemical that is not hard to find or weaponize, experts say.
If chlorine was used in the attack as alleged by the western alliance then much or maybe even all of it will have evaporated in the ten days since the incident. Its ally Moscow has accused the West and medics in Douma of staging the attack.
"Experts from the chemical weapons committee enter the town of Douma", state news agency SANA wrote, referring to the Organisation for the Prohibition of Chemical Weapons (OPCW).
OPCW director general Ahmet Uzumcu said his nine-strong, all-volunteer team had reached Damascus, but so far "the team has not yet deployed to Douma".
Similarly, France's foreign ministry said in a statement that it is "very likely that proof and essential elements are disappearing from this site". The Russian government accused the United Kingdom of staging the attack, which the British government says is "ludicrous".
Mr Saleh said bodies had been buried quickly because of heavy bombardment and the location of graves was kept secret to prevent any doctoring of evidence.
The U.N. Security Council is due to be briefed on Tuesday, at the request of Russian Federation, on the situation in Syria's northern city of Raqqa, where Islamic State was defeated a year ago by USA -backed forces, and the Rubkan camp for displaced Syrians near the country's border with Jordan and Iraq.
The inspectors will also be looking for other evidence, such as canisters, rocket or bomb fragments, impact sites and craters and will take photographs and video of them. "Everything else is secondary", the White Helmets member said.
He and two other residents accused the rebel Army of Islam of carrying out the attack.