The US group approached AstraZeneca, which has a market value of £51.5 billion ($86.6 billion), within the last week and could make a public declaration of its interest in a takeover as early as this week, said people familiar with the matter.
Going public would be a move designed to put pressure on AstraZeneca's board to engage in discussions. Pfizer first telegraphed its interest four months ago, when it asked the UK group to consider a takeover. The overture was rebuffed, however, and no formal talks took place.
The exact value Pfizer is placing on AstraZeneca could not be ascertained, but people familiar with the matter said a bid would be likely to come in at more than $100 billion. This would make it the biggest pharma deal since Pfizer's $111.8 billion takeover of Warner-Lambert in 2000.
People close to the situation cautioned that there was no guarantee of a public declaration by Pfizer.
The move comes at a time of renewed corporate activity in the sector as drugmakers look to deploy large cash piles and cheap debt to strengthen their positions in an increasingly competitive market.