NTT DoCoMo Inc. said Tuesday it has kept open its option to take over AT&T Wireless Services Inc., the third-largest mobile phone service provider in the United States.

While AT&T Wireless decided to solicit proposals to take it over, NTT DoCoMo has reserved the right to make a proposal "on a fair basis," a spokesman at the biggest Japanese mobile phone operator said.

He was commenting on an agreement between the two companies announced earlier in the day in which NTT DoCoMo is waiving the right to receive information on proposals made by other companies to acquire AT&T Wireless.

NTT DoCoMo has the right as the biggest shareholder in AT&T Wireless with a 16 percent stake.