"We [DPR] have to acknowledge the correct and impartial approach [of the OSCE] regarding the recent tragedy near Volnovakha, where the OSCE mission has at least documented the direction, from which the shell was fired," Pushilin said.
Earlier on Saturday, the OSCE Special Monitoring Mission (SMM) in Ukraine released a report on the recent bus attack that killed 12 people.
The investigation included comprehensive analysis of two specific blast craters, including the crater located 98 feet from the side of the bus. The SMM assessed that all the examined craters were caused by rockets fired from a north-north-eastern direction.
On Tuesday, 12 people were killed and 18 were injured when a bus was attacked close to a Ukrainian military checkpoint near the village of Volnovakha in the eastern Donetsk region.
Kiev had claimed that the bus was hit by rockets fired from the east, from the direction of a town where local independence fighters were allegedly present at the time. The DPR authorities have stressed they were not involved in the attack.
The ongoing shelling in east Ukraine's Donetsk Region shows Kiev's inability to negotiate, Denis Pushilin said.
"I have to admit that [the shelling] we have been hearing since the nighttime — since yesterday — echo the Ukrainian side's non-negotiability," Pushilin said at a press conference.
The deputy speaker also claimed Ukraine's "lack of unity" and inability to "arrive at independent decisions" lead to the latest escalation in the conflict.
Armed skirmishes in southeast Ukraine intensified over the past two weeks in violation of a truce reached in September 2014 in the Belarusian capital of Minsk. Both sides place the blame for ceasefire violations on each another, citing scores of attacks taking place in Donetsk and its airport, which has become a hotspot in the latest outburst of violence.
Kiev's military operation to suppress independence supporters in southeast Ukraine dates back to April 2014. According to Friday's estimates by a World Health Organization (WHO) spokesperson, the death toll from the armed conflict exceeded 4,800 people.