The U.S. Treasury Department on Thursday offered to accept a global minimum corporate tax of at least 15% during international negotiations, a rate significantly below its proposed 21% minimum for U.S. multinational firms.

The department said the proposal was made during an Organization for Economic Cooperation and Development (OECD) tax steering group meeting on base erosion and profit shifting. The group is aiming reach broad agreement this summer to rework rules for taxing multinational corporations and big technology companies such as Alphabet Inc (GOOGL.O) and Facebook Inc (FB.O).

"Treasury proposed to the steering group that the global minimum tax rate should be at least 15%," the department said in a statement. "Treasury underscored that 15% is a floor and that discussions should continue to be ambitious and push that rate higher."