The U.S. military says Yemen's Shi’ite Houthi rebels shot down a U.S. government-operated drone earlier this month with "assistance" from Iran.

"A U.S. MQ-9 was shot down over Yemen by what we assess to be a [Houthi] SA-6 surface to air missile on Jun 6," Lieutenant-Colonel Earl Brown, a spokesman for the U.S. Central Command, said in a statement on June 16.

Brown added that the altitude at which the drone was shot down marked "an improvement over previous [Houthi] capability, which we assess was enabled by Iranian assistance."

U.S. ally Saudi Arabia is leading a military coalition backing Yemen's government in its four-year war with the Huthi rebels. The conflict is seen as a proxy war between Riyadh and regional rival Tehran.

Brown also said that on June 13, "a modified Iranian SA-7 surface-to-air missile attempted to shoot down" a U.S. MQ-9 drone over the Gulf of Oman in an effort to disrupt surveillance of the "IRGC attack" on Kokuka Courageous -- one of two oil tankers attacked in the area that day.

The United States and its allies blame the attacks on the two vessels on Iran, which denies any involvement.

The Pentagon has released a video claiming to show patrol boats from Iran's Islamic Revolutionary Guards Corps (IRGC) removing what appeared to be an unexploded mine from the side of the Kokuka Courageous.