There's probably a difference between under-reporting and propaganda.
Any idea of the difference in scale between the casualty figures or the systematic process with which one is being carried out by 2 State actors? If you want a pop at the Western media, why not look at the coverage of the Saudi-led intervention in Yemen, which it is claimed, is supported by the UK by the wicked sale of armaments. The Times featured it at the weekend.
It still doesn't come anywhere near the eradication of rebel-held Aleppo, so I can't see how you reasonably claim this to be propaganda.