SAN FRANCISCO (MarketWatch) -- The Android mobile operating system passed the Apple iPhone in terms of U.S. market share during the first quarter, according to a report issued Monday by the NPD Group.

According to the study, the Android OS -- developed by Google Inc. GOOG, +0.01% -- ended the period with a domestic market share of 28%. That's up from about 20% in the December quarter, and is due mostly to strong sales of handsets such as the Droid and Droid Eris at Verizon Wireless VZ, +0.42% , according to the report.

The Motorola Droid. Motorola

The iPhone from Apple Inc. AAPL, +1.50% saw its U.S. share remain relatively flat at 21%. The leader in the U.S. remains Research In Motion RIMM, whose BlackBerry family of "smart phone" devices has about 36% of the market, according to NPD data.

"Verizon is putting considerable effort behind promoting Android to its customers," said NPD analyst Ross Rubin of the latest report.

A big part of the boost for the Android platform came from the launch of the Droid handset from Motorola MOT in late 2009. The device racked up strong sales and helped the platform's market share surge from below 5% in the third quarter to end the year around 20%, according to NPD data.

The data are supported somewhat by a similar report issued from IDC on Friday. Looking at the smart-phone market by vendor, IDC found that Motorola had about 11.7% market share in North America in the first quarter, while HTC -- another big vendor of Android handsets -- had 9.2%. Combined, those numbers were ahead of the 17.2% market share attributed to Apple. See full story on IDC's smart-phone report.

Rubin said much of the growth for Android is likely attributable to the fact that -- before the launch of the Droid -- Verizon customers had few alternatives to the iPhone at AT&T T, +0.74% .

"Prior to launch of Droid, there had not been a lot of modern, touch-optimized smart phones available at Verizon," Rubin said, who added that the majority of smart-phone sales at a carrier typically come from customers who are already signed up to that carrier.

Verizon is expected to put heavy promotion behind the latest Android device -- the Droid Incredible from HTC -- for the next several months.

The data may also indicate pressure on Apple to expand its base of carriers for the iPhone. The device is still exclusive to AT&T in the U.S. market, despite persistent speculation that it might expand to Verizon later this year. The company has never commented on any of these rumors.