Amari will meet with ministers from the Association of Southeast Asian Nations (Asean) in a bid to iron out remaining differences.

"There is no signing but we sincerely hope we can have an agreement in principle," said the source, who asked not to be named.

"We want to have a common understanding on the modality for tariff reduction on trade in goods, but even at this moment I cannot say we are 100 percent sure," he told AFP by telephone from the Brunei capital Bandar Seri Begawan.

"We are still negotiating on the table."

Japanese and Asean working-level officials who met in Tokyo last month for the seventh round of their talks on a so-called comprehensive economic partnership agreement (EPA) said they hoped to strike an accord in Brunei on merchandise trade.

Such a free trade pact, which is expected to eventually include services and investment cooperation, would form the core of the EPA which would encompass broader economic ties.

But the source said that senior officials who had been meeting in Brunei since Tuesday ahead of Friday's ministerial gathering had not been able to reach an agreement.

"Honestly speaking, there is some uncertainty," he said.

Japan has already signed free trade agreements with four Asean members -- Thailand, Malaysia, Singapore and the Philippines -- but wants an accord that will cover all 10 Asean members.

Indonesia and Brunei had expressed agreement "in principle" on a free trade deal with Japan, but at least one less developed Asean member had expressed some concern, the source said.

"They are a bit worried about our exports," he said, mentioning auto parts.

Japan and Asean agreed to liberalise more than 90 percent of merchandise trade in value within 10 years after an EPA took effect.

Tokyo has said it planned to eliminate tariffs on some 92 percent of its imports from Asean bloc in the 10-year period, while Asean aimed to remove tariffs on more than 90 percent of its imports from Japan.

Asean's other members are Cambodia, Laos, Myanmar and Vietnam.