* Drills across half of country as U.S.-Iran tensions high
* Involve 8,000 troops, test planes, defence systems -media
* Iran jets fired on U.S. drone over Gulf Nov. 1, U.S. says
By Yeganeh Torbati
DUBAI, Nov 12 (Reuters) - Iran launched military drills across half the country on Monday, warning it would act against aggressors less than a week after Washington accused Iranian warplanes of firing on a U.S. drone.
The manoeuvres take place this week across 850,000 square kilometres (330,000 square miles) of Iran's northeast, east, and southeast regions, Iranian media reported.
About 8,000 elite and regular army troops will participate, backed by bombers and fighter planes, while missile, artillery and surveillance systems will be tested, they said.
Played out against a backdrop of high tension between the United States and Iran over Tehran's nuclear programme, the "Velayat-4" manoeuvres will involve the biggest air drills the country has ever held, Iran's English-language Press TV said.
Western experts have challenged some of Iran's military assertions, saying it often exaggerates its capabilities.
"These drills convey a message of peace and security to regional countries," Shahrokh Shahram, spokesman for the exercises, told Press TV on Monday. "At the same time they send out a strong warning to those threatening Iran."
Last week, the U.S. Pentagon said Iranian planes opened fire on an unarmed U.S. drone over international waters on Nov. 1. Iran said it had repelled "an enemy's unmanned aircraft" violating its airspace.
Senior researcher Pieter Wezeman of the Stockholm International Peace Research Institute said an international arms embargo imposed against Iran meant the country was using outdated military equipment, including aircraft.
"The U.N. embargo on supplies of most types of major weapons to Iran is blocking Iranian military modernisation," he said. "Iran is more and more falling behind in military terms."
But London-based defence analyst Paul Beaver said Iran's military should not be underestimated, describing it as "a pretty impressive organisation".
"They are busy out there modifying, adapting, doing things to their technologies. They have made the most of what they have," he said.
Western powers have imposed sanctions on Iran's oil trade to press it to halt nuclear work they fear is aimed at developing the capability to build nuclear bombs. Iran denies the charge, saying its atomic activities are purely for peaceful purposes.
The United States and Israel have not ruled out military action if diplomacy fails to resolve the dispute.
Although the Iranian air drills come just days after the Pentagon's announcement, the exercises appear to have been planned well in advance.
In September Farzad Esmaili, commander of the army's air defence force, said Iran was planning a large-scale air drill in coming months.
Various radar and other fixed, tactical and airborne surveillance systems would participate, Esmaili told state news agency IRNA on Thursday. The exercise will also test bombers, refuelling planes and unmanned aircraft, Esmaili said.
Iranian media said on Monday that F-4, F-5, F-7, and F-14 fighters would take part.
Shahram told IRNA the drills would also focus on improving coordination between Iran's military and the elite Islamic Revolutionary Guard Corps.
On Sunday, Revolutionary Guards Brigadier General Amir Ali Hajizadeh said Iran believed the U.S. drone was gathering intelligence on oil tankers off its shores.
Mohammad Ali Jafari, the Guards' top commander, said his forces had acted well in repelling the drone. "If such intrusions take place in the future, we will protect our airspace," Jafari said on Sunday, according to Press TV.
Iranian officials have threatened to strike U.S. military bases in the region and target Israel if the country is attacked.
Iran has carried out a number of military simulations this year, including the "Great Prophet 7" missile exercises in July.
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