Pfizer disclosed Monday that AstraZeneca in January rejected an offer of about $99 billion to acquire the British company and that AstraZeneca last weekend rebuffed a second effort to engage in merger talks.
AstraZeneca on Monday said Pfizer's bid "significantly undervalued" the British company.
Reports in financial media, citing unnamed sources, said Pfizer plans to bid $106 billion or thereabouts, with some of the stories saying the pharmaceutical giant could offer a larger share of the offer in cash as opposed to Pfizer stock.
Pfizer declined to comment on the reports.
The biggest US drug maker by revenue has said the deal would combine research pipelines in oncology and other key disciplines. Pfizer also cited the potential for huge spending cuts and a lower tax bill.
Pfizer's attempt to buy AstraZeneca comes amid a wave of deal-making in the pharmaceutical sector as drugmakers face big sales declines due to patent expirations.