MOSCOW - Russia on Friday criticised the world's chemical weapons watchdog for not sending experts to the site of an alleged chemical attack in Syria, backing up President Bashar al-Assad's regime.

"We consider it unacceptable to analyse events from a distance," Foreign Minister Sergei Lavrov said at a press conference in Moscow with his counterparts from Syria and Iran. Lavrov said Assad's opponents had "in essence" given guarantees for the Organisation for the Prohibition of Chemical Weapons to visit the location where at least 87 people died, but the watchdog was refusing to send them. "They say still that it is not very safe, but they cannot put forward convincing arguments," Russia's top diplomat said.

Russia has rejected accusations from the West that its Assad's forces were responsible for a chemical attack and has lashed out at the US for its cruise missile strikes last week against a Syrian air base.

The OPCW said Thursday that a fact-finding mission was analysing samples gathered from "various sources" and that allegations a chemical attack took place in the Syrian rebel-held town of Khan Sheikhun were "credible". Lavrov said Russia, Iran and Syria have demanded a "thorough, objective and unbiased investigation" under the auspices of the OPCW, insisting it must use "independent experts", including from Moscow.

Russia on Wednesday vetoed a Western-backed UN resolution demanding that the Syrian government cooperate with an investigation into the alleged attack, blocking Security Council action against Moscow's ally for the eighth time  Russia and Iran are the firmest allies of Assad's regime and have deployed forces to the war-torn country to back him in the country's brutal six-year conflict.

Lavrov was meeting with his counterparts from Syria and Iran in a show of support for Damascus after US Secretary of State Rex Tillerson visited Moscow earlier this week.  Lavrov repeated the Kremlin's condemnation of the US missile strike in Syria and said Washington was seeking "excuses for regime change". "These attempts will not succeed, this will not happen," he said.

 Meanwhile, hundreds of civilians and fighters who have been under crippling siege for more than two years left four Syrian towns in fleets of buses Friday under a delayed evacuation deal. Men, women and children packed onto the buses, expressing despair at leaving their homes with no way of knowing when they might return. "When I first went onto the bus, I broke down from sadness, I fell on the ground and they had to help me," said Abu Hussein, a resident of the government-held town of Fuaa. "I just couldn't bear it.

The deal to evacuate government-held Fuaa and Kafraya, and rebel-held Madaya and Zabadani is the latest in a string of such agreements through Syria's six-year civil war.  They have been touted by the government as the best way to end the fighting but rebels say they are forced out by siege and bombardment.

Critics say deals are permanently changing the ethnic and religious map, but in an exclusive interview with AFP on Wednesday President Bashar al-Assad insisted the evacuations were only temporary and people would return once the "terrorists" had been defeated.

The evacuation of the four towns had been due to start on April 4, but implementation of the deal brokered by rebel supporter Qatar and regime ally Iran late last month was repeatedly delayed. At least 80 buses left Fuaa and Kafraya in Idlib province in the northwest, an AFP correspondent in rebel-held territory said.

They arrived at a marshalling point in Rashidin, west of government-held second city Aleppo, followed by 20 ambulances.

The Syrian Observatory for Human Rights said 5,000 people had left the two towns, including 1,300 pro-government fighters.

The pro-government Al Watan newspaper said 3,000 more would follow in a second convoy on Friday night.

Dozens of rebel fighters, including from Al-Qaeda's former Syria affiliate Fateh al-Sham Front, stood guard at the marshalling point, the AFP correspondent reported.

As the buses waited, the Red Crescent distributed water to waiting passengers.

A civilian travelling in one of the evacuation buses from the rebel-held towns of Madaya and Zabadani said the operation began at around 6:00 am (0300 GMT). "Most of the passengers are women and children who started gathering yesterday evening and spent the night in the cold waiting," Madaya resident Amjad al-Maleh told AFP by telephone.

He said that rebel fighters among the evacuees had been allowed to keep their light weapons.

The evacuees were all searched when they reached government-held territory. "It is a very bad feeling when you see those who besieged you and killed you with hunger and bombardment right in front of you," Maleh said. "Madaya cried today - the ones who stayed and the ones who left."

The Observatory said 2,200 people from Zabadani and Madaya had left, among them 400 rebels.

Madaya doctor Mohamed Darwish was among the evacuees. "People feel like they are being slapped in the face. They are shocked," he said.

More than 30,000 people are expected to be evacuated under the deal, which began on Wednesday with an exchange of prisoners between rebels and government forces.

All 16,000 residents of Fuaa and Kafraya are expected to leave, heading to government-held Aleppo, the coastal province of Latakia or Damascus.

Civilians from Madaya and Zabadani will reportedly be allowed to remain if they choose. Those who opt to leave will head to rebel-held territory in Idlib.

The four towns are part of a longstanding agreement reached in 2015 that requires aid deliveries and evacuations to be carried out to all areas simultaneously.

But access has been limited, with food and medical shortages causing malnutrition, illness and even death among besieged residents.

The UN says 4.72 million Syrians are in so-called hard-to-reach areas, including 600,000 people under siege, mostly by the Syrian army, but also by rebels or the Islamic State group.

There has been a series of evacuations in recent month, mostly around the capital Damascus but also from the last rebel-held district of Syria's third city Homs.

The overwhelmingly Sunni rebels have charged that Assad's regime is deliberately forcing civilians to leave to alter the country's sectarian map.

But in Wednesday's interview, the president told AFP that it was the rebels who were driving people from their homes.

"We wish that everyone could stay in his village and his city, but those people like many other civilians in different areas were surrounded and besieged by the terrorists, and they've been killed on (a) daily basis, so they had to leave," he said.

"But of course they're going to go back to their cities after the liberation.

"Talking about demographic changes is not in the sake or in the interest of the Syrian society when it's permanent. As long as it's temporary, we wouldn't worry about it."