A number of disgruntled Syngenta shareholders have set up an investor group in an effort to push for change at the Swiss agricultural chemicals group.

Basel-based Syngenta is under pressure to boost shareholder returns after fending off a $47 billion takeover approach from U.S. seeds developer Monsanto in August.

The investor group, which calls itself the Alliance of Critical Syngenta Shareholders, launched a website where other stakeholders can sign up and also sent a letter to Syngenta Chairman Michel Demare.

"The Alliance urges the board to thoroughly evaluate all options for value creation without prejudice and to ensure that promises are kept, financial targets are met and shareholders are informed in a timely and open manner," it said in a statement on Saturday.

A Syngenta spokesman said the company had noted the group's letter.

The shareholder group said it was made up of private and institutional investors in Syngenta who were unhappy with the company's management and performance in recent years.

It did not disclose the number of members it has, or the size of their Syngenta holdings.

Syngenta said this week third-quarter sales fell by more than expected as the weak currency in Brazil, its second-largest market, eroded the value of overseas business.