NEW DELHI - India's thermal coal imports from Russia are expected to fall for the first time in four months in September, two research consultancies said, potentially resulting in lower revenues for Moscow at a time it is mobilising more troops to fight in Ukraine.

Indian consultancy Coalmint expects September thermal coal imports from Russia to decline 30% from August to 1.4-million tonnes, it said in a note to clients.

London-based analytics firm DBX Commodities estimates Indian shipments of the power generation fuel to fall to 1.5-million tonnes this month from 1.9-million tonnes in August, CEO Alexandre Claude told Reuters on Thursday.

Energy exports to India and China have been crucial revenue sources to sanctions-hit Russia, but have also helped bridge supply gaps and place a lid on surging global fuel prices.

Russian President Vladimir Putin on Wednesday called up 300 000 reservists to fight in Ukraine, a major escalation and the first mobilisation of this kind since World War Two, which would require higher spending.

The value of India's coal imports from Russia since troops marched into Ukraine on February 24 rose to about $2.4-billion as of Wednesday, said a source who requested anonymity, citing nonpublic Indian government data. This is a four-fold increase compared with shipments during the same period last year.

Traders and analysts say logistical issues have resulted in lower imports in September by India from Russia. Coal shipments from Russia had risen in July and August despite a decline in overall Indian imports of the fuel, making Russia India's third largest coal supplier. 

A longer waiting period for vessels, higher traffic congestion in the Baltic region due to Europe's scramble for energy supplies and difficulties in securing insurance were resulting in lower Indian imports from Russia, Coalmint said.

Sanctions have cut many European countries off from supply of power-generation fuels from Russia, the continent's top supplier, resulting in a scramble for coal, natural gas and electricity in Europe amid record high prices.

"India's coal imports from Russia are likely to be in a tight range in the coming months with vessel availability likely to be an issue with the start of winter restocking by various countries, starting October," Coalmint said in the note.

DBX expects India's total thermal coal imports to fall to about 13-million tonnes from 15-million tonnes in August, Claude said.

"(Overall Indian) thermal coal imports have been declining since June as the heatwave then started to recede and the economy started to falter," he said.