Former CIA station chief Dan Hoffman discusses the growing threat between the U.S. and Iran.

Washington (CNN) -- In the hours before the attack on the two tankers in the Gulf of Oman on Thursday, the Iranians spotted a US drone flying overhead and launched a surface-to-air missile at the unmanned aircraft, a US official told CNN.

The missile missed the drone and fell into the water, the official said.

Prior to taking fire, the American MQ-9 Reaper drone observed Iranian vessels closing in on the tankers, the official added, though the source did not say whether the unmanned aircraft saw the boats conducting an actual attack.

CNN has not seen any imagery from the US drones.

Iran has strenuously denied any involvement in the attack.

The same official also said in the days prior to the attack, a US Reaper drone was shot down in the Red Sea by what is believed to be an Iranian missile fired by Houthi rebels.The Pentagon had tough words for Iran on Friday as the US continues to assert its claim that Tehran was responsible for the attack on the two oil tankers in international waters after releasing video footage its says shows an Iranian patrol boat removing an unexploded mine from one of the vessels' hulls.

"We're making sure that General McKenzie and the central command has the resources and the support that they need to conduct their missions," acting Defense Secretary Patrick Shanahan said, adding that the US continues to work on building an international consensus that Iran was behind the attack >>>