The United States has challenged the Japanese government over moves to ramp up exports of coal-fired power technology and to offer cheap loans to lure buyers, according to a U.S. source with direct knowledge of the matter.

Japan's shipments of the equipment soared to nearly $8 billion last year as it looks to boost infrastructure exports, defying U.S. calls for developed nations to stop investing in foreign coal projects, to curb greenhouse gas emissions.

While the issue is unlikely to blow up into a major dispute, it could taint the two allies' typically close relationship on energy and comes ahead of a November gathering of the OECD where members are expected to discuss coal-fired power funding.