A day after U.S. drugmaker Allergan rejected its initial takeover approach, Canada's Valeant Pharmaceuticals International said it would "improve" the unsolicited $47 billion offer. The Canadian firm added that it would announce the new cash and stock offer price at a May 28 webcast.

At the same time, it will discuss details of its plans for the US company's business. "We will not stop our pursuit of this combination until we hear directly from Allergan shareholders that you prefer Allergan's 'stay the course plan' to a combination with Valeant," Valeant CEO Michael Pearson said in a letter to shareholders.

Earlier, Allergan, the manufacturer of the popular anti-wrinkle treatment Botox, had claimed that Valeant's business model was unsustainable.

Valeant's signal of an improved offer near the end of the month sets up an intense two-week period for the two sides to sway shareholders, some of whom own stock in each company.

Together with Allergan's biggest shareholder, Pershing Square Capital Management - headed by activist shareholder Bill Ackman - Valeant said it may conduct a referendum of Allergan investors on whether the target company's board should negotiate with Valeant.

On April 22, Allergan put in a so-called "poison pill" to slow Ackman from increasing his nearly 10 % stake in the company.