Iran Monday launched a massive four-day defense exercise even as tensions mounted in the Gulf after the U.S. accused that country of firing at a drone last week.

Iran’s air force is holding the Velayat-4 defense maneuvers over an 850,000-square-meter area in the eastern half comprising northeast, east and southeast.  

Dubbed as the biggest air drill in the country, Velayat-4 will demonstrate Iranian air defense capabilities.

"Various kinds of fixed, mobile, and tactical long-range radars, and fixed, tactical and airborne electronic surveillance systems will participate in this exercise," said Farzad Esmaili, who heads Iran's air defense headquarters, according to state news agency IRNA.

Iran frequently conducts various defense drills to underline its military strengths to deter its enemies.  

"For those who are thinking of aggression against the Islamic Republic of Iran, the exercise should be seen as a strong warning," drills spokesperson Brigadier General Shahrokh Shahram told the Press TV.

Various air defense systems including electronic warfare instruments, long-range radar systems, missiles, wiretapping, observation posts and artillery units are participating in the military drill. The exercise will also see the participation of Iran’s fighter jets, tanker and drone planes.  

“The F-4, F-5, F-7 and F-14 jet fighters as well as Mirage and Sukhoi aircrafts and surveillance planes took off from eastern and southern air bases flying over the sky of the military drill’s area,” IRNA reported.

The drill was apparently pre-planned even though it is happening a week after the Iranian attack on the U.S. drone. According to an ISNA report, the drill was scheduled for early October, but was postponed to November for unknown reasons.

The tensions between Tehran and Washington escalated last week after the Iran army’s attack on the drone.

Iran claimed that the unmanned aircraft had violated its airspace and had been spying on its oil tankers, local media reported.  

"Iran's air defense systems and IRGC fighter jets did their job and forced the aircraft out of Iran's skies," Mohammad Ali Jafari, commander of the Guards, said Sunday, according to the Press TV.

However, the U.S. has refuted Iran’s claims. It said that the drone had been flying in the international territory and had not violated the Iranian territory.