TOKYO (AFP) - A Japanese minister who was the country's top negotiator for a huge trans-Pacific trade deal was accused of corruption on Thursday (Jan 21), piling pressure on Prime Minister Shinzo Abe ahead of parliamentary elections this year.

Weekly magazine Shukan Bunshun claimed on Thursday that Economy and Fiscal Policy Minister Akira Amari, who also serves as Japan's chief negotiator for the Trans-Pacific Partnership (TPP), and his staff accepted a 12 million yen (S$148,000) "bribe" from a construction firm.

The allegations against a key ally of the prime minister come ahead of upper house elections in July and as the government looks to ratify the TPP, a massive multination deal of which Japan has been a key player.

Opposition lawmakers pressed Mr Amari in Parliament over the report, with the minister responding that he had only just read it, adding: "I will thoroughly investigate the matter and fully carry out my responsibility to explain what happened."

The minister acknowledged that he had met with officials from the construction company, which Shukan Bunshun did not identify, but said he could not recall the details.

The magazine said the company was looking for Mr Amari's help in settling problems with a quasi-public housing agency over damage caused by a road project.

It cited a company official as saying the firm had wined and dined the minister's staff and had given them gifts and envelopes stuffed with cash, including on two occasions handing a total of 1 million yen to Mr Amari himself.

Shukan Bunshun said the official had recorded conversations and held other records, including expenses logs and photocopies of cash given to the minister.

The magazine, like many of its weekly counterparts, has gained a reputation for feisty journalism. In 2014, one of its rivals published a report on corruption allegations against the industry minister that eventually led to her resignation.

Mr Abe said of the allegations in Parliament: "Minister Amari said he will swiftly investigate the issue and will offer full explanations."

"I firmly believe he will do just that."

Mr Abe's Liberal Democratic Party government, which came to power three years ago, has seen several ministers resign over a variety of scandals.

The latest claims are a potential blow to the premier as he seeks the cooperation of other parties to secure a two-thirds majority in the Upper House needed to help amend Japan's Constitution.

Mr Abe is also looking to have the TPP, which is to be formally signed in New Zealand next month, ratified by Japanese lawmakers during the current session of Parliament.

The pact aims to create the world's biggest free-trade area, bringing together 12 Pacific Rim countries, including the United States, Japan and Australia.