President Mahmoud Ahmadinejad insisted that the plant, near the holy city of Qom, did not breach UN regulations and said it was open for inspection by UN experts.
But leaders of the US, Britain and France accused Tehran of keeping the plant secret in breach of UN rules.
They raised the prospect of new, tougher sanctions against Iran if it does not fully co-operate with global powers.
Iran is due to meet with representatives of the so-called P5 + 1 on 1 October to discuss a range of issues, including its nuclear plans. The P5 + 1 groups together the five permanent members of the UN Security Council - the US, Russia, China, the UK and France - along with Germany.
Iran's state-run al-Alam and Press TV channels said on Sunday that the short-range missiles test-fired during military exercises were of Tondar-69 and Fateh-110 type.
BBC Tehran correspondent Jon Leyne
It's not the first time the Shahab missile has been tested but it's still likely to provoke condemnation in Western capitals.
Rightly or not this will be seen as a gesture of defiance by Iran.
It comes two days after the existence was revealed of a previously undisclosed nuclear plant, thought the test itself is likely to have been planned well in advance.
It aired footage of a missile launch in desert-like terrain, without giving specifics on the range of the missiles or other details.
Western defence experts believe the Fateh, or Conqueror, missile has a range of up to 170km (100 miles), while the Tondar (Thunder) can travel up to 150km (93 miles).
Iran says the Shahab-3 (Meteor-3), which was first tested in July 2008, can fly some 2,000km, which makes it capable of hitting Israel and US bases in the Gulf.
However, Western defence experts believe it could strike targets up to 1,300km (807 miles) away.
The experts say Iran's claim of the longer range could indicate that the 2008 test involved a newer, modified version of the Shahab 3, perhaps the rumoured Shahab 3b, which Iran says would have a range of up to 2,500km (1553 miles).
Many believe Iran is attempting to develop its Shahab missiles to carry nuclear warheads, but analysts say perfecting this ability is a complex process.
Iran's Revolutionary Guards announced on Saturday they would begin war games - codenamed The Great Prophet IV - to "improve the Islamic Republic armed forces' defence capabilities".
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