“We are warning our Ukrainian colleagues against attempts to once again return to the scenario of force,” Foreign Minister Sergei Lavrov said at a press conference in the Russian capital, Moscow, on Wednesday.

Referring to a rocket strike on a bus transporting civilians on Tuesday, which left 12 people dead, he said, “The shooting has become massive.”

Lavrov further urged the government in Kiev to launch a “thorough and objective” investigation into the deadly incident.

Both the Ukrainian government and the pro-Russia forces accused each other of being behind the rocket attack.

“There are… several versions, we need to examine them,” Lavrov said, calling on the Organization for the Security and Cooperation in Europe, which has a mission in Ukraine, to elucidate the deadly incident.

A statement on the website of the Russian Foreign Ministry, meanwhile, denounced the attack as a “provocation” that seeks to destabilize the ceasefire in Ukraine’s east signed last September in the Belarusian capital, Minsk.

Both the Ukrainian military and pro-Russia forces have frequently violated the ceasefire deal. The two sides often accuse each other of firing stray rockets and carrying out attacks, which claim the lives of civilians.

Ukraine’s mainly Russian-speaking regions of Donetsk and Lugansk in the east have witnessed deadly clashes between pro-Russia forces and the Ukrainian army since Kiev launched military operations to silence the pro-Russia protests in mid-April 2014.

More than 4,700 people have been killed in the fighting, the UN says.