The conflicting accounts could put pressure on both sides for more details on U.S. reconnaissance and Iranian counter-measures.
They also point to other questions, including how Iran could manage to snatch the Boeing-designed ScanEagle drone without noticeable damage to its light-weight, carbon-fiber body or whether the aircraft could be from another Gulf country that deploys it.
There is even the possibility the drone is authentic but was plucked from the sea after a past crash and unveiled for maximum effect amid escalating tensions over U.S. reconnaissance missions - including a Predator drone coming under fire from Iranian warplanes last month.
But unlike the larger Predator, which can carry weapons and sophisticated surveillance systems, the much smaller ScanEagle collects mostly photographic and video images using equipment with little intelligence value, experts said. One called the craft a "large seagull" with cameras.
Monitoring of Gulf air and sea traffic is considered of high importance for the U.S. military. Iran has taken steps to boost its naval and drone capabilities, unsettling Washington's Gulf Arab allies. Iran also has threatened in the past to try to block the strategic Strait of Hormuz - the route for one-fifth of the world's oil - in retaliation for Western sanctions over Tehran's nuclear program.
"We had warned American officials not to violate our airspace. We had formally protested such actions and had announced that we protect our borders," state TV quoted Foreign Minister Ali Abkar Salehi as saying.
Washington denies it has crossed into Iranian airspace, but Iran's definition of its jurisdiction could be far broader. State-run Press TV said any surveillance of Iran was considered "a violation of territory."
Asked about Iran's assertion that it had captured a U.S. drone, White House press secretary Jay Carney said "we have no evidence that the Iranian claims are true."
Cmdr. Jason Salata, a spokesman for the U.S. Navy's 5th Fleet in Bahrain, said there are no Navy drones missing in the Middle East.
"The U.S. Navy has fully accounted for all unmanned air vehicles operating in the Middle East region," said Salata. "Our operations in the Gulf are confined to internationally recognized waters and airspace."
He noted that some ScanEagles operated by the Navy "have been lost into the water" over the years, but there is no "record of that occurring most recently."
Other countries in the region, including the United Arab Emirates and Kuwait, also have ScanEagle drones in their fleets. It's used by other militaries, but not among Iran's close allies.
The Iranian announcement did not give details on the time or location of the claimed drone capture.
It's certain, however, to be portrayed by Tehran as another bold challenge to U.S. reconnaissance efforts in the region.
Last month, the Pentagon said a drone came under Iranian fire in the Gulf but was not harmed. A year ago, Iran managed to bring down an unmanned CIA spy drone possibly coming from Afghanistan.
Iran claimed it captured the drone after it entered Iranian airspace. A report on state TV quoted the navy chief of Iran's powerful Revolutionary Guard, Gen. Ali Fadavi, as saying the Iranian forces caught the "intruding" drone.
"The U.S. drone, which was conducting a reconnaissance flight and gathering data over the Persian Gulf in the past few days, was captured by the Guard's navy air defense unit as soon as it entered Iranian airspace," Fadavi said. "Such drones usually take off from large warships."
Al-Alam, the Iranian state TV's Arabic-language channel, showed two Guard commanders examining what appeared to be a ScanEagle drone with no visible damage or military markings on its gray fuselage or wings.
The semiofficial Fars news agency, which is close to the Guard, said it was the captured drone, a propeller-driven craft with a 10-foot (three-meter) wingspan that's sent aloft from a pneumatic launcher from even a small vessel - undermining Iranian claims that it needs a warship to be deployed.
The drone, built by Boeing subsidiary Insitu Inc., typically would have no high-value intelligence and is used mostly for aerial photographs and video.
"With a ScanEagle, you just throw it off your boat to have a look over the horizon. It's not, like, a major system," said aviation expert Paul E. Eden. "In military chest-beating terms, the U.S. is likely to just laugh at the Iranians for making so much of having captured one."
Eden also expressed skepticism that the ScanEagle could have been shot down, calling it akin to a "large seagull" because it's slow and very small, making it a tricky target.
"If you did hit it with any anti-aircraft weapons, there wouldn't be much left," said Huw Williams, a drone expert at Jane's International Defense Review.
In the Iranian TV footage, the two men then point to a huge map of the Persian Gulf in the background, showing the drone's alleged path of entry into Iranian airspace. "We shall trample on the U.S.," was printed over the map in Farsi and English next to the Guard's emblem.
If true, the seizure of the drone would be the third reported incident involving Iran and U.S. drones in the past two years.
Last month, Iran claimed that a U.S. drone had violated its airspace. The Pentagon said the unmanned aircraft came under fire - at least twice but was not hit - and that the Predator was over international waters.
The Nov. 1 shooting in the Gulf was unprecedented, and further escalated tensions between the United States and Iran, which is under international sanctions over its suspect nuclear program. Tehran denies it's pursuing a nuclear weapon and insists its program is for peaceful purposes only.
In late 2011, Iran claimed it brought down a CIA spy drone after it entered Iranian airspace from its eastern borders with Afghanistan and Pakistan. The RQ-170 Sentinel drone, which is equipped with stealth technology, was captured almost intact. Tehran later said it recovered data from the drone.
In the case of the Sentinel, after initially saying only that a drone had been lost near the Afghan-Iran border, American officials eventually confirmed it had been monitoring Iran's military and nuclear facilities. Washington asked for it back but Iran refused, and instead released photos of Iranian officials studying the aircraft.
Iran meanwhile, has claimed advancements in drone technology.
In November, Iranian media reported that the country had produced a domestically made drone capable of hovering. Earlier, Iran said it obtained images of sensitive Israeli bases taken by a drone that was launched by Lebanon's Hezbollah and downed by Israel.
Iran also claimed other drones made dozens of apparently undetected flights into Israeli airspace from Lebanon in recent years. Israel has rejected the Iranian assertions.
Murphy reported from Dubai, United Arab Emirates. Associated Press writers Cassandra Vinograd in London and Julie Pace in Washington contributed to this report.