BEIRUT (Reuters) - Supreme Leader Ayatollah Ali Khamenei said Wednesday that Iran needs to boost its offensive military capabilities.

"In order to secure our population, our country and our future we have to increase our offensive capabilities as well as our defensive capabilities,” he said at a military expo in Tehran where a number of top military officials gathered, according to the Islamic Republic News Agency (IRNA).

The Shi'ite leader spoke a week after a U.S. Navy ship fired warning shots toward an Iranian fast-attack craft that had approached two U.S. ships, according to a Pentagon spokesman. 

The Pentagon also said that Iranian vessels had harassed a U.S. warship near the Strait of Hormuz, a vital oil and gas shipping channel, early last week. 

For its part, the Iranian military accused the United States of sending a drone into its air space on Monday, according to the Tasnim news agency. It said the drone left Iranian air space after a warning. 

President Hassan Rouhani, a pragmatist, has presided over a limited thaw in Iran's relations with the West, with a 2015 nuclear deal with six world powers his most notable achievement. But Iran's military and security services that answer to the hardline Khamenei continue to see Western states as adversaries.

Khamenei also said that expanding defensive capabilities was necessary “so that oppressive powers feel threatened”, an allusion to the United States and key European allies.

At the expo, held to highlight the Islamic Republic’s defense industry, there were displays and presentations of Iranian-made missiles and drones as well as the Bavar 373 missile defense system, according to IRNA.

(Reporting by Babak Dehghanpisheh; editing by Mark Heinrich)