David Cameron is facing accusations that he has become a "diplomatic irrelevance" as France and Germany spearhead a new push to broker a peace plan in Ukraine.
Britain's former top Nato commander General Sir Richard Shirreff said the UK should be playing a major role in the efforts to resolve the crisis amid fears that it could lead to an all-out war in Europe.
He told BBC Radio 4's Today programme: "This is the most serious crisis to have faced Europe, arguably, since the Soviet invasion of Czechoslovakia in 1968. There is a threat of total war.
"The UK is a major Nato member, it is a major EU member, it is a member of the UN Security Council, and it is unfortunate that the weight that the British Prime Minister could bring to efforts to resolve this crisis appear to be absent."
Video footage recorded from Kramatorsk, deep inside Ukrainian-government controlled territory, shows rockets landing on civilian areas.
Video footage appears to show the moment a shell strikes a community soup kitchen in eastern Ukraine.
A mother in eastern Ukraine buries her son - yet another casualty in a civil war that is killing civilians with appalling randomness.