Nippon Oil Corp. and Nippon Mining Holdings Inc. will delay a plan to merge in October because they need time to submit documents to U.S. regulators, sources said Thursday.

The two companies underestimated the time they would need to reconcile the differences between Japanese and U.S. accounting standards, and may take several months to complete submissions, said officials involved in the discussions who asked not to be identified before an announcement Friday.

The U.S. Securities and Exchange Commission requires the filings from companies that are more than 10 percent owned by U.S. shareholders, they said.

The merger, which will create Japan's third-largest company by revenue, needs to be approved by stockholders of Nippon Oil and Nippon Mining at their annual general meetings in June. The combined entity will help save ¥60 billion in costs annually as the global slump in fuel demand forces the country's refiners to reduce output and shut plants.

"It's an issue for Japanese companies," said Taisuke Igaki, an attorney specializing in mergers and acquisition at Kitahama Partners. The SEC requires businesses to submit additional information in an F-4 filing before shareholders vote on matters, including mergers, he said.

Spokesmen at Nippon Oil and Nippon Mining declined comment.

U.S. shareholders hold 31 percent of Nippon Mining, Japan's largest copper producer, and 9.7 percent of Nippon Oil.