British drugs company AstraZeneca said its bigger U.S. rival Pfizer was making an "opportunistic attempt" to buy the company for a price that didn't reflect the value of its "exciting pipeline" of new drugs.

In response to an appeal from Pfizer for it to enter negotiations, AstraZeneca said Pfizer's announcement on Tuesday contained no new proposal nor substantive new information.

(Reporting by Paul Sandle; editing by Ben Hirschler)