The reports from sources such as the Nikkei, Japan's main business daily, and Kyodo News agency come as MetroPCS is in final negotiations to merge with Deutsche Telekom's T-Mobile USA. Those two firms announced they had agreed to merge earlier this month, but Softbank aims to convince MetroPCS shareholders that it can provide a better deal, the Nikkei said.

Softbank and Sprint have confirmed they are in talks, with Sprint saying they could lead to a "substantial investment." Softbank is looking to purchase at least two-thirds of its U.S. counterpart, according to reports.

Both operators are third in their respective countries, although Softbank could soon vault to number two in Japan, after a deal it announced earlier this month to acquire domestic rival eAccess for US$2 billion to help expand its mobile network.

The Japanese firm is mulling a total investment of over $30 billion in Sprint and MetroPCS, which would rank as the largest foreign acquisition of a foreign company, the Nikkei said.

A Softbank spokesman said the company had no comment on the reports of a MetroPCS deal.