Japan's SoftBank Corp. said on Tuesday that it has bought an additional 0.73 percent of Sprint Corp's common stock, increasing its stake in the U.S. wireless operator to 79.07 percent.

SoftBank, which is run by founder Masayoshi Son, said the share purchases, made through subsidiary Galaxy Investment Holdings, were intended to increase the company's ownership in Sprint to about 80 percent.

The company, which bought about 78 percent of the No. 3 U.S. mobile network operator in July after a bitter takeover battle, said Galaxy intends to buy about 2 percent of Sprint shares.

It did not say why it is increasing its stake. Sprint shares have risen almost 22 percent since July 8.