MILAN (Reuters) -A leading Chinese investor in Italian tyre maker Pirelli said on Thursday a proposal the company had put forward to solve problems that having a Chinese shareholder pose for its U.S. expansion plans were "seriously unfair".

Without providing details on the proposal, Pirelli had said on Wednesday that its Chinese shareholder Sinochem had rejected it. Pirelli makes over 20% of its revenues in North America.

"The proposal was rejected by us because (it is) potentially disruptive for Pirelli and overall seriously unfair and unbalanced for all Pirelli shareholders ... other than Camfin," Marco Polo International Italy, the vehicle through which Sinochem invests in Pirelli, said in a statement.

In a statement on Thursday, Pirelli stressed that the proposals presented were in the interests of the company and respectful for all its shareholders.

The tyremaker and its second-largest shareholder, Italy's Camfin, have said Sinochem's shareholding was hindering the tyremaker's ambitions to expand in the U.S., where some lawmakers are opposed to approving projects backed by Chinese companies.

"Pirelli remains open to exploring solutions that will allow its full compliance with the rules also in the American market and will continue to do all within its power to protect the company's development in such a strategic market as that of the United States," it said in its statement.

The firm said on Thursday that there have been no alternate proposals from Marco Polo.

Sinochem had earlier reiterated its "firm support for the long-term sustainable development of Pirelli", saying it would continue to work with its shareholders, management and the relevant authorities to help Pirelli's growth and development.

(Reporting by Valentina Za and Gnaneshwar Rajan, editing by Gavin Jones and Alan Barona)