British spooks fear Russian forces will use phosphorus weapons on Mariupol's citizens as the battle for the city rages on.
Vladimir Putin recently unleashed white phosphorus weapons in the eastern city of Kramatorsk.
White phosphorous is used to cause rapid fires due to the aggressive way it combusts with oxygen.
The use of the chemical is not banned under international convention when it is used as a smokescreen, but as an incendiary weapon in civilian areas, it is forbidden under the Geneva convention.
The Ministry of Defence warned this morning of the "possibility" of Russian troops potentially using the weapon in Mariupol.
In an intelligence update for April 11, the ministry said: "Russian forces prior use of phosphorous munitions in the Donetsk Oblast raises the possibility of their future employment in Mariupol as fighting for the city intensifies."
Mariupol, with its strategic location on the coast of the Sea of Azov, has been battered by Russian strikes since the invasion began on February 24.
Last week, dramatic footage showed the moment Ukrainian soldiers appeared to wipe out a Russian armoured personnel carrier in Mariupol.
Another video from the scene shows the bodies of at least six Russian infantry troops.
Putin has withdrawn his troops from Kyiv after fierce resistance from Ukrainian forces and large losses in troops and military equipment.
However, Putin is not pulling out of his unjust war as he appears to continue his shelling in other parts of the country.
It comes as Putin has turned to personnel from those who were discharged from service in 2012 to replace military personnel with the Russian death toll at almost 20,000 according to Ukraine.
Putin is also said to be recruiting troops from Moldova, which has been classed as an occupied territory of Russia by the Council of Europe.
Over the weekend, satellite images showed an eight-mile-long Russian convoy is making its way towards under-siege Ukraine marking a new phase in the conflict.
Images from space showed the mass of military vehicles and artillery travelling in the direction of the eastern part of the war-torn country.
Maxar technologies took the satellite images which showed the heavy military build-up as it made its way through Velykyi Burluk.
The military column is headed toward eastern Ukraine according to experts who reckon it is bound for Luhansk and Donetsk.
Last month the deputy head of Kyiv's police said white phosphorus munitions were deployed over Kramatorsk.
Videos were taken on phones and by an ITV film crew showed a glittering light substance falling from the sky onto the city below.
Oleksiy Biloshytskiy shared a video of material burning fiercely on the ground as it was touched with a spade.
“Another use of phosphorus ammunitions in Kramatorsk,” he says in the video.
