Anatoly Klyan, who worked for state-controlled Channel One, was killed in what his employers said was an overnight attack by Ukrainian forces on a bus carrying journalists and soldiers' mothers.
Russia said the death of the 68-year-old undermined faith in Kiev's commitment to a ceasefire and its desire to end weeks of conflict peacefully.
The Ukrainian authorities, for their part, say the separatists have repeatedly breached the ceasefire with attacks on military posts and Ukrainian bases since it was declared on June 20.
Klyan was the third journalist from a Russian state network to be killed since pro-Russian separatists began an uprising in east Ukraine in April.
An Italian journalist and his Russian translator were also killed in May.
Klyan was shot in the stomach while accompanying a group of soldiers' mothers on their way by bus to a Ukrainian military unit "to meet their sons and take them home," Channel One said, apparently meaning that the women wanted the troops to withdraw and stop fighting the separatists.
It said the bus withdrew after coming under fire as it approached the base, but Klyan died in a round of automatic rifle fire after getting off it.
Moscow renewed calls for an end to Ukraine's military operation in the eastern regions of Donetsk and Luhansk, where separatists have seized state buildings and weapons arsenals.
"The death of a Russian journalist again convincingly demonstrates that Ukrainian forces clearly do not want a de-escalation of the conflict in the east of the country," the Russian Foreign Ministry said in a statement.
"They are hampering the already fragile ceasefire," it said.
Russia's Investigative Committee later opened a criminal investigation into the shooting, Interfax news agency said.
Ukrainian Defence Ministry spokesman Bohdan Senyk said he had no information about the violence but that Russian journalists had worked among separatists in the past, making it difficult to identify them as reporters.
"Russian journalists have more than once worked among terrorist groups and those who send them bear responsibility for them," he said.
Sporadic violence has continued in eastern Ukraine despite a ceasefire declared by President Petro Poroshenko on June 20 to allow for peace talks with the rebels.
Poroshenko said last Thursday that 18 Ukrainian servicemen had been killed since he announced the truce, including nine who died in the downing of a helicopter by separatists near the rebel stronghold of Slaviansk on June 24.
The ceasefire was due to expire at 10 p.m. (1900 GMT) on Monday but the Ukrainian, Russian, French and German leaders were due to discuss the crisis in Ukraine by telephone before then.
Source: Google News