Further violence by Russian-backed separatists in Ukraine will lead to a "grave deterioration" in relations between the European Union and Moscow, Philip Hammond warned.
Ukraine has said that intercepted communications prove that separatists were responsible for firing rockets that pounded the south-eastern city of Mariupol and killed at least 30 people.
The Foreign Secretary called on Russia to end its support for the separatists and exert its influence over the rebel leadership to bring an end to the attacks.
US president Barack Obama put the blame squarely on Moscow and said his country would work with European partners to "ratchet up the pressure on Russia."
EU foreign affairs chief Federica Mogherini later announced that EU foreign ministers would hold an extraordinary meeting in Brussels on Thursday to discuss Ukraine amid continued tensions between the West and Russian president Vladimir Putin.
The Foreign Secretary said the latest attacks showed a "blatant refusal" to abide by the peace plan agreed following talks in Minsk last year.
Mr Hammond said: " I condemn in the strongest terms yesterday's Russian-backed separatist attacks in Mariupol killing at least 30 innocent civilians, including children, and wounding many others. My thoughts are with the victims and their friends and families.
"I am deeply concerned about the significant escalation in violence in east Ukraine over the past week. Recent announcements by the separatist leaders of further offensives, and their blatant refusal to abide by the ceasefire, raise serious questions about the commitments they made at Minsk.
"I call on Russia to stop its material support to the separatists immediately, and use its considerable influence over the separatist leadership to stop these indiscriminate attacks, and fully abide by the commitments they made at Minsk.
"Russia will be judged by its actions, not words. If the escalation in fighting continues, with tragic consequences for the local population, this will lead to a further grave deterioration in relations between the EU and Russia."