A fast-growing number of software applications offers doctors the chance to view X-rays or monitor heart rates from iPads or smartphones, prompting U.S. health officials on Tuesday to propose government oversight for some of the more complex health apps.

A portion of the apps perform tasks that mimic the work of medical devices and could cause harm if they failed to perform as promised, Food and Drug Administration officials said.

New FDA guidelines state that in some cases software developers would have to show their apps worked as well as non-mobile devices for the same use. The agency would have to provide clearance for those apps before they could be sold.

FDA officials said they recognized the potential benefits from mobile medical apps and aimed to encourage future development. At the same time, we need to make sure these things are also safe and effective, Dr. Jeffrey Shuren, head of the agency's medical device center, said in an interview.

Medical apps are sold for devices such as Apple Inc's iPad and iPhone, Blackberry phones sold by Research In Motion, and phones run with Google Inc's Android software.

Only a small number of the thousands of mobile apps would face FDA scrutiny under new draft guidelines, agency officials said. They fall into two categories: apps that serve as an accessory to an FDA-regulated device, and apps that turn a mobile platform into a medical device.

Examples are apps that allow a doctor to view X-ray or MRI images on a smartphone or tablet computer, or see vital signs displayed on a patient's bedside monitor from another location.

Others may allow connection of heart sensors on a patient's chest to a mobile device, or provide a way to use a smartphone like a stethoscope.

One concern is that smaller screen sizes and lower contrast ratios could affect the quality of images on a mobile device.

Regulators hope the guidelines provide clarity on their expectations and will spark new app development, said Bakul Patel, policy adviser in the FDA device center. The agency aims to strike that fine balance between promoting innovation and assuring safety and effectiveness, Patel said.

Apps for consumers that count calories or provide medical reference materials would not draw FDA oversight.

The FDA already has cleared a handful of medical apps after developers sought the agency's blessing. In February, for example, an app that lets doctors view CT, MRI and other scans on Apple's iPhone or iPad won FDA clearance.

The agency is taking public comment the proposed guidelines for 90 days.

(Editing by Philip Barbara)

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