WASHINGTON — The stealth C.I.A. drone that crashed deep inside Iranian territory last week was part of a stepped-up surveillance program that has frequently sent the United States’ most hard-to-detect drone into the country to map suspected nuclear sites, according to foreign officials and American experts who have been briefed on the effort.

Until this week, the high-altitude flights from bases in Afghanistan were among the most secret of many intelligence-collection efforts against Iran, and American officials refuse to discuss it. But the crash of the vehicle, which Iranian officials said occurred more than 140 miles from the border with Afghanistan, blew the program’s cover.

The overflights by the bat-winged RQ-170 Sentinel, built by Lockheed Martin and first glimpsed on an airfield in Kandahar, Afghanistan, in 2009, are part of an increasingly aggressive intelligence collection program aimed at Iran, current and former officials say. The urgency of the effort has been underscored by a recent public debate in Israel about whether time is running out for a military strike to slow Iran’s progress toward a nuclear weapon.

In a recent speech, President Obama’s national security adviser, Tom Donilon, hinted at secret efforts by the United States to keep watch on Iran’s nuclear program.