Russia has accused the Kiev government of violating the ceasefire in eastern Ukraine, saying it has intensified its military offensive against rebels.
The accusation came as Ukraine announced the introduction of passport controls around areas held by pro-Russian separatists and heavy artillery fire erupted in Donetsk.
"It is obvious today that these agreements have been crudely violated by the Ukrainian side," the Russian Foreign Ministry said on Thursday.
"In place of de-escalation, Kiev has intensified the hostilities in the southeast of the country with the use of heavy equipment, which has led to thousands of casualties and large-scale destruction."
It is obvious today that these agreements have been crudely violated by the Ukrainian side.
Ukraine's border guards service said anyone crossing in or out of rebel areas will have to present a passport.
Foreigners will "be sent to filtration points to determine the purpose of their visit" and will have "to show a passport or the required visa," a statement said.
The rule was explained as a security measure to seal off the separatist region in the east, where artillery fire violated a tattered ceasefire deal on multiple occasions early on Thursday.
It was not possible to verify which side was responsible for the fire, AFP reported.
However, the passport regime will create a de facto internal border, underscoring the Russian-backed separatists' success in carving out two self-declared republics centred on Donetsk and Luhansk.
Arseniy Yatsenyuk, Ukraine's prime minister, said on Wednesday that he would like to discuss the Ukrainian crisis in the "Geneva format", referring to a four-way meeting in April which included the US and the EU.
The accords reached in Geneva, Switzerland, called on all "illegal armed groups" in Ukraine to surrender.
However, since then the rebels have declared independence in Donetsk and Luhansk regions and more than 4,000 people have been killed as Ukrainian forces attempt to retake the areas.
The fragility of the situation was underlined on Thursday when the Kiev government and the rebels traded blame over the shelling of a school the day before, which left two teenagers dead and four others injured.
Andriy Lysenko, a Ukrainian military spokesman, said that preliminary information indicates the shells flew into the schoolyard in Donetsk from a location controlled by separatist forces.
But the rebels blamed Ukrainian forces for Wednesday's deadlyattack, saying government troops indiscriminately target residential areas in eastern Ukraine.
Lysenko said that in the latest shelling across the conflict zone, nine Ukrainian soldiers had been wounded. The separatists rarely release casualty figures.
He also repeated accusations that "Russian units and mercenaries" continued to cross into Ukraine.
Russian denies directly aiding the rebels, although Russia soldiers and equipment have been frequently spotted by journalists in the conflict zone.