(CNN) Shelling and artillery fire continued in Syria's besieged rebel-held enclave of Eastern Ghouta on Tuesday, interrupting a five-hour "humanitarian pause" ordered by Russian President Vladimir Putin.

Russia, a key ally of the Syrian regime, on Monday ordered a daily pause in hostilities -- from 9 a.m. to 2 p.m. local time -- to begin on Tuesday.

The order called for a humanitarian corridor to allow civilians to leave the enclave on the edge of the capital Damascus, but made no mention of whether food and medical supplies would be able to enter the area.

But as of Tuesday morning, Putin's proposed pause appeared to have had little impact. Within minutes of when the ceasefire was meant to start, activists on the ground reported shelling and artillery fire from pro-regime positions, killing at least one person.

Russia and the Syrian regime accused the rebels of shelling humanitarian corridors, preventing civilians from leaving.

Putin's order came just days after Saturday's United Nations resolution calling for a 30-day ceasefire. It wasn't clear from the resolution when that ceasefire was meant to start, or how it would be enforced.

Kremlin spokesman Dmitry Peskov told reporters on Tuesday that Russia sees the humanitarian pause in Eastern Ghouta as part of the effort to implement the UN ceasefire resolution. He added that the daily pause would be in place for at least a month.

Whether the Russian ceasefire order will succeed, where the UN has not, remains to be seen.

What is happening in Eastern Ghouta?

A renewed offensive in Eastern Ghouta has claimed the lives of 568 civilians since mid-February, according to the UK-based Syrian Observatory for Human Rights. The area, home to some 400,000 people, has been under siege for years.

Eastern Ghouta is one of the last major rebel-held areas of the country. Observers fear the enclave could face a similar fate to eastern Aleppo, which was all but destroyed in a government offensive in December 2016.

The regime's capture of that city marked a turning point in the war, with Syrian President Bashar al-Assad taking back control of all four major cities in the country -- with the help of Russia.

Russia's intervention in the country's civil war in 2015 -- with troops and weaponry -- has helped tilt the balance in Assad's favor, with the push for Eastern Ghouta now more intense than ever.

'We don't trust them'

Shortly after Russia's proposed ceasefire took effect, activists on the ground described hiding beneath buildings from shelling and artillery fire.

"We don't trust the Russians on the humanitarian pause between 9 a.m. to 2 p.m. local, because we have seen in the past they don't commit to these humanitarian pauses," activist Bilal Abu Salah told CNN.

There have been many ceasefires in Syria's past, dating back to 2011, when war first broke out:

In 2011, Syria signed an Arab League proposal to stop the fighting between government forces and protestors, but violence continued.

In February 2016, the US and Russia coordinated a partial ceasefire near Aleppo and Raqqa but that fell apart days later.

In September 2016, the US and Russia brokered a pact to pause violence after months of back-and-forth talks to allow for humanitarian aid in cities enduring Syrian government airstrikes.

In December 2016, Russia, Turkey, Iran and the Syrian government declared a ceasefire throughout much of the country.

Last year, a "four zone" ceasefire was declared for areas including Idlib province, Eastern Ghouta and southern Syria.

Last month, Turkey disregarded calls from France and other countries for a ceasefire in the northern Syrian area of Afrin where Turkish forces are battling Kurdish militias allied with the US, as well as in Idlib and Ghouta.

'The proof will be in the silence'

As the fighting continues, aid workers were waiting to enter Eastern Ghouta, said a spokesman for the UN Secretary General Stephane Dujarric.

"We stand ready as soon as the conditions are safe for truck drivers, humanitarian workers to roll into these areas," Dujarric said during the daily UN press briefing. "We need to ensure that there are no roadblocks, whether physical or administrative. Whether five hours is enough or is not enough is a difficult question to answer. Five hours is better than no hours."

Asked if rebel groups had indicated whether they would go along with the Russian proposal of five hours, Dujarric said: "The proof will be in the silence. Once the guns go silent we know."

The International Committee of the Red Cross also is ready to deliver humanitarian aid, said Robert Mardini, the ICRC's Middle East director, adding that "any initiative that would give some respite to civilians from these relentless hostilities is positive."

But distributing aid in five hours is impossible, he said.

"We have a long experience of bringing aid across front lines in Syria, and we know that it may take up to one day to simply pass checkpoints, despite the previous agreement of all parties. Then you need to offload the goods," Mardini said.

Chlorine gas claims

Russia's ceasefire order comes as the White House condemned reports of the Syrian regime's alleged use of chlorine gas in Eastern Ghouta Sunday.

"Syria is terrorizing hundreds of thousands of civilians," White House press secretary Sarah Sanders said Monday. "The regime's use of chlorine gas as a weapon only intensifies this. The United States calls for an immediate end to offensive operations."

Sixteen patients, including six children, were treated in the hospital "suffering from symptons indicative to exposure from chemical compounds," according to the Syrian American Medical Society.

Several people in the suburb were treated for apparent exposure to chlorine gas, Syrian opposition groups said, as air strikes and artillery fire from the regime continued on Sunday.

The opposition-run Rural Damascus Health Directorate said the people were admitted to medical facilities showing signs that were consistent with exposure to "toxic chlorine gas."

Images obtained by CNN show men and children receiving treatment, some of them using oxygen masks.

A child is given treatment for what the Syrian American Medical Society described as exposure to chemical compounds.

The White Helmets, a volunteer rescue group, said in a tweet that one child was killed in a chlorine gas attack in the city.

CNN is unable to independently verify claims that chlorine was used as a weapon in Eastern Ghouta.

Both sides of the conflict have in the past accused each other of using chlorine as a weapon, and the government has repeatedly denied claims that it has done so.

Russian Foreign Minister Sergey Lavrov told reporters that allegations of a chlorine attack were a "hoax," and attacked the White Helmets for promoting what he described as "fake stories." The Russian Defense Ministry alleged that "terrorists" hiding in Eastern Ghouta have chemical substances that "might be used for provocations."

UK weighs in, Russia implies fake news

Russia's claims of fake news continued Tuesday, as UK Foreign Secretary Boris Johnson said Britain would consider involvement in Syria if there was "incontrovertible evidence" of chemical attacks.

"What we in the West need to ask ourselves is, 'Can we allow the use of chemical weapons, the use of these illegal weapons to go on unapproved, unchecked, unpunished?' And I don't think that we can," Johnson said in a BBC interview.

But the Russian Embassy in London suggested reports of chemical attacks in Eastern Ghouta were the work of the White Helmets volunteer rescue group producing fake news.

It tweeted a Times of London newspaper report on Johnson's comments Tuesday, with the caption: "Does this imply that there are already fake White Helmets videos in production at @PinewoodStudios?" referring to the British film studios.

Terrorist groups not included

The UN ceasefire resolution, which passed unanimously on Saturday, noted that the truce "shall not apply to military operations against the Islamic State," al Nusra and other groups associated with them, or those deemed terrorist organizations by the UN Security Council.

A member of the Syrian Civil Defence carries an injured child following air strikes on Eastern Ghouta on February 7.

The main rebel units actively holding territory in Eastern Ghouta are the Islamist Jaish al Islam and Faylaq al Rahman, which have taken part in peace negotiations in the past. According to activists, there are small pockets of Hayat Tahrir al Sham, an al-Qaeda affiliate, still in the area.

Because the Syrian regime and its allies Russia and Iran view Eastern Ghouta as under "terrorist" control, they have indicated that the area is also not subject to the ceasefire.