It comes after more than 60 Syrian soldiers were killed by US/British forces
The Syrian Army has declared the end of a ceasefire brokered by the US and Russia.
The fragile deal, which was agreed last week, was put under extra strain when airstrikes mistakenly killed around 60 Syrian soldiers on Saturday.
America apologised for bombing a base near Deir al Zor airport, insisting the intended target was Islamic State fighters.
While Britain's Ministry of Defence has also admitted that the UK took part in a US-led coalition airstrike which hit Syrian forces.
A spokesman said: "We can confirm that the UK participated in the coalition air strike south of Dayr az Zawr on Saturday, and we are fully co-operating with the coalition investigation.
"The UK would not intentionally target Syrian military units. It would not be appropriate to comment further at this stage."
Syria's President Assad called the airstrikes a "flagrant aggression".
Denmark said two of its F-16 jets took part in the raid, and Australian aircraft were also involved.
The ceasefire did not exclude attacks on IS or other jihadist groups.
Russia's foreign ministry accused the US-led coalition of being on the "boundary between criminal negligence and direct connivance with Islamic State terrorists".
The ceasefire has expired as vital aid supplies remain stuck at the border with Turkey. There is no immediate indication that the truce will be extended.
The so-called "regime of calm" was declared by the Syrian army last Monday following a ceasefire brokered by the United States and Russia.
There have been accusations from all sides of numerous violations of the truce - which was the second this year.
"The Syrian army had announced a freeze on fighting until Sunday night, but as Russia announced an extension, it will end on Monday at 1600 GMT," a diplomatic source told the AFP news agency.
The ceasefire has so far failed to allow aid to be delivered to the besieged city of Aleppo.
The UN said a 20-truck aid convoy destined for the east of the city with enough supplies to feed tens of thousands is still stuck in Turkey, a week after arriving at the border.
"I am pained and disappointed that a United Nations convoy has yet to cross into Syria from Turkey, and safely reach eastern Aleppo, where up to 275,000 people remain trapped without food, water, proper shelter or medical care," said UN Under-Secretary General for Humanitarian Affairs Stephen O'Brien.
The UN has said it does not have sufficient security guarantees from all sides to deliver the supplies.
It also wants to deliver to other areas which are hard to reach, but said it had not received necessary permission from the Syrian government.
Moscow has accused rebel groups of violating the truce and said Washington would be responsible if it broke down.
The main battlefronts have seen a marked reduction in violence, but clashes broke out late last week in Hama and Homs, as well as east of Damascus.
On Sunday, the first airstrikes since the ceasefire began hit rebel-held districts in Aleppo, killing one woman.
Russian President Vladimir Putin questioned US commitment to the ceasefire, claiming Washington was not prepared to break with "terrorist elements" battling President Assad's forces.
The United Nations has previously said that humanitarian convoys were unable to move due to a delay in getting permits from the Syrian government.
"It is particularly regrettable because...we are losing time," UN Special Envoy for Syria Staffan de Mistura told a press briefing in Geneva.
"These are days which we should have used for convoys to move with the permit to go because there is no fighting".
According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), Syria is one of the most complex and dynamic humanitarian crises in the world today.
Since March 2011, more than a quarter of a million Syrians have been killed and over one million have been injured.
Some 4.8 million Syrians have been forced to leave the country, and 6.5 million are internally displaced, making Syria the largest displacement crisis globally.
In 2016, an estimated 13.5 million people, including six million children, are in need of humanitarian assistance. Of these 5.47 million people are in hard-to-reach areas, including close to 600,000 people in 18 besieged areas.