Western countries and North Atlantic Treaty Organisation accused Russia of sending troops over the border into Ukraine, but Russia maintains any Russian fighters there are "volunteers".
Western nations have called for restraint after Ukraine raised the alarm over a buildup of Russian forces near its border and violence rose along the line between Kiev's troops and separatists in Ukraine's east.
The Kremlin said in its readout of the Merkel phone call that "Vladimir Putin noted provocative actions by Kyiv which is deliberately inflaming the situation along the line of contact".
"Every day of the war claims the lives of fearless, patriotic people who must build the future of Ukraine", the President said, the press service of the Head of State informs.
The United States and European Union are watching reports of Russian troop movements on Ukraine's border with increasing concern.
Russian Federation is going into election mode ahead of parliamentary polls in September and domestic irritants such as jailed Kremlin critic Alexei Navalny make it easier for Moscow to focus on an external enemy, he said.
"Kyiv speaks about reaching peace via joining a military alliance - an oxymoron the Ukrainian leader is sending to the media space with a serious face".
Images released by his office showed Zelensky in the trenches clad in a helmet and bulletproof vest, handing out awards to Ukrainian soldiers and shaking their hands.
"Moscow has also threatened a military response if Sweden or Finland decides to join North Atlantic Treaty Organisation; according to NATO's secretary-general, Russian exercises have included simulated nuclear strikes against Sweden", wrote Richard Sokolsky, a nonresident senior fellow in Carnegie's Russia and Eurasia Program. Both sides have since accused each other of violating it.
Clashes between Ukrainian troops and the Moscow-backed rebels inside Donbass have also increased in recent months.
Zelensky, who has urged North Atlantic Treaty Organisation to speed up his country's membership into the alliance to support Ukraine, said he had visited positions where "the largest number of violations" of a ceasefire had been recorded, the presidency said in a statement.
In this analysis, Zelensky lost whatever maneuvering room he still had when the coronavirus pandemic blasted the Ukrainian economy.
Zelensky further alienated Moscow with anti-corruption investigations and pro-Western reforms that reduced Russian influence in Kyiv, his efforts to keep Russia's annexation of Crimea alive as an worldwide issue, and his insistence on ignoring the Donbass separatist leadership as puppets as he insists on direct negotiations with their Russian patrons.
A meeting of a trilateral working group on Ukraine's rebel-held Donbass region ended on Wednesday without any tangible results, said Russia's envoy to the group, Boris Gryzlov. A war could damage Russia's investments in Crimea, as well as major deals with Europe such as the Nord Stream 2 pipeline.
"There is a lot at stake, Russian President Vladimir Putin will continue to pursue the politics of blackmail and pressure, even if he is not, in principle, prepared to engage in hostilities", says the analyst.