By Diane Bartz

Jan 20 (WASHINGTON) - Apple Inc filed a $1 billion lawsuit against supplier Qualcomm Inc on Friday, following a U.S. government lawsuit which accused the chip maker of using anti-competitive tactics to maintain its monopoly of a key semiconductor in mobile phones.

Qualcomm is a major supplier to both Apple and Samsung for "modem" chips that help phones connect to wireless networks. Together the two accounted for 40 percent of Qualcomm's $23.5 billion in revenue in its most recent fiscal year.

In the lawsuit, Apple accused Qualcomm of overcharging for its chips and for refusing to pay some $1 billion in promised rebates for chip purchases. Apple said in its complaint that Qualcomm withheld the rebates because of Apple's discussions with South Korea's antitrust regulator, which fined Qualcomm for what it called unfair practices in patent licensing.

Qualcomm did not immediately respond to a request for comment on the Apple lawsuit.

On Tuesday, the U.S. Federal Trade Commission filed a lawsuit against Qualcomm, saying that the San Diego-based company used its dominant position as a supplier of certain phone chips to impose "onerous" supply and licensing terms on cellphone manufacturers like Apple and to weaken competitors.

Qualcomm said in a statement Tuesday that it would "vigorously contest" the FTC complaint.

(Reporting by Diane Bartz and Stephen Nellis; Editing by Alan Crosby)