The Bushehr tests, which began 10 days ago, are expected to take between four and seven months, Mr Aghazadeh said.
The reactor was first started by a German company under the Shah in 1975, but revolution and war stalled work there until the Russians took it on in 1995.
Operations of the 1,000-megawatt light-water reactor have been long delayed, though it is expected to start generating power later this year.
The BBC's Jon Leyne, one of the international correspondents to visit the site on Wednesday, said that although some equipment was still wrapped in plastic, Russian engineers were already at work amid blinking lights.
He also noted heavy security at Bushehr, including machine-gun nests and anti-aircraft guns.
"The construction stage of the nuclear power plant is over, we are now in the pre-comissioning stage, which is a combination of complex procedures," said Sergei Kiriyenko, head of Russia's nuclear agency.
"Today we visited the reactor and saw fuel rod imitators being loaded in the active zone," he added.
The official Irna news agency reported that a date for the plant to become operational would be announced during the pre-commissioning process.
Iran is currently defying Security Council resolutions ordering it to suspend the enrichment of uranium. It says it is simply doing what it is allowed to do under the Nuclear Non-Proliferation Treaty (NPT).
The country has pledged not to break its obligations under the NPT and will not use the technology to make nuclear weapons.
However, the Iranian authorities hid their uranium enrichment programme for 18 years, and the Security Council says they should stop enrichment and certain other nuclear activities until their peaceful intentions can be fully established.
Our correspondent says Iran would have to take the dramatic step of expelling international inspectors and Russian engineers for it to use the Bushehr plant to make nuclear bombs.
There is no suggestion of this happening at the moment, he adds.
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