The European Union says escalating attacks on government-controlled areas in eastern Ukraine violate a February peace deal with pro-Russia separatists.
“The Minsk Agreements must be implemented in good faith, starting with full observation of the ceasefire and genuine withdrawal of heavy weapons,” the EU said.
Ukraine on Monday accused rebels of carrying out the heaviest shelling since the truce was agreed.
Russia denies any role in the fighting.
However, Ukraine and the West say there is a growing body of evidence pointing to direct involvement from Moscow.
Rebel leaders have denied firing on Ukrainian positions and accuse government forces of violating the ceasefire themselves some 40 times over a 24-hour period.
Some of the worst fighting was near the village of Starohnativka, 50km (31 miles) north of the strategic port of Mariupol.
Ukraine says pro-Russia forces launched dozens of attacks in a number of locations on Monday and Tuesday.
“The renewed escalation of the conflict raising the number of casualties, as a result of attacks on several government controlled areas today and in the night of 10 August on Starohnativka, violates the spirit and the letter of the Minsk Agreements,” the EU’s External Action Service said late on Tuesday.
In its statement, the EU condemned an arson attack on Sunday on armoured vehicles belonging to the monitors from the Organisation for Security and Cooperation in Europe (OSCE) as well as the fact that the monitors had been caught in the crossfire in Shchastya, in Luhansk, and Shyrokyne, near Mariupol.
These incidents “put in danger the crucial role” of OSCE officials in “monitoring and verifying the implementation of the Minsk agreements,” it said.
Meanwhile a London-based think tank released a report on Wednesday warning that an increase in military exercises by Russia and Nato in and around the Baltic states and Poland raised the threat of armed conflict in Europe.
The European Leadership Network (ELN) said that the war exercises fed “uncertainty” and heightened the risk of “dangerous military encounters”.
Relations between Russia and the West have been in crisis since Russia’s annexation of Crimea from Ukraine last year.
Around 7,000 people have been killed since the Ukrainian conflict started and more than one million people have been displaced.
The ceasefire agreed in Minsk in February has not brought an end to the conflict, but it has resulted in a lower level of fighting.
There are fears now that the violence could erupt once again into a full-scale war.