LUKoil is expected triple its gas production to about 23 billion cubic meters in 2020 to become the fourth-largest producer in Russia.

The agency said it the plot was in "advanced planning stages" but gave no further info on how close the men got to carrying it out.

The behind-the-scenes negotiations follow the first day of a major peace conference in Switzerland which ended in bitter divisions.

“We repeat our call for both police and protesters to exercise restraint and refrain from violence and provocation,” Krivokapic said.