Japan's antitrust watchdog will question Amazon Japan over its plan to offer reward points on all products sold on its site, on the suspicion that small merchants will be forced to shoulder the cost of the points, sources close to the matter said Tuesday.

The Japan Fair Trade Commission plans to start a full-fledged survey from Wednesday to determine whether the e-commerce operator gave smaller businesses and individual sellers the chance to negotiate the terms of the new system starting in May, they said.

Failing to do so could be deemed as an abuse of Amazon's superior bargaining position over the other parties, prohibited under the antimonopoly law. The company declined to comment on the issue.