Officials in Australia and Japan on Monday expressed skepticism and confusion over a pledge by Prime Minister Tony Abbott to give an Australian state-owned shipbuilder the option to tender for a major submarine contract, heightening the uncertainty around the project.

Sources have said Australia is strongly considering buying a version of the 4,000-ton Soryu-class submarine built by Japan's Mitsubishi Heavy Industries and Kawasaki Heavy Industries to replace its aging Collins-class fleet.

But Abbott on Sunday promised shipbuilder ASC Ltd. the option to bid for the next-generation submarines, worth as much as 40 billion Australian dollars ($31 billion), in an attempt to shore up support ahead of a challenge to his leadership from within the ruling Liberal Party. He survived the revolt during a vote earlier Monday.