Iran's military is on standby in case of an attack against its nuclear facilities, the country's top army commander has said.
Iran denies claims that it wants to build nuclear weapons
Gen Mohammad Salimi said training had been suspended to concentrate on patrols close to potential targets.
Iran suspects that Israel may attack its nuclear sites because of fears that Tehran is developing nuclear weapons.
Iran says it held at least 10 people for allegedly spying on atomic sites for Israel and the US in the past year.
"The air force has been ordered to protect the nuclear sites, using all its power," Gen Salimi told a government newspaper.
"All our forces including land forces, anti-aircraft [and] radar tactics are protecting the nuclear sites and an attack on them will not be simple," he added.
Gen Salimi's comments came amid claims that US military planners have run simulations of a complex attack on Iran's nuclear sites.
The US magazine Atlantic Monthly reported that the Pentagon had simulated a three-stage attack on Iran, beginning with operations against suspected nuclear bases.
At an Iranian cabinet meeting, Intelligence Minister Ali Yunesi said those arrested for spying had been working in Iran for the CIA and Mossad.
An official said that three of those arrested had been working within the state's nuclear programme itself.
In August, Tehran announced the arrest of a number of spies accused of supplying information to other states.
"More than 10 nuclear spies were arrested during the current Iranian year," Mr Yunesi is quoted as saying by the official Irna news agency.
He said the 10, whom he accused of working for the CIA and Mossad, were arrested in Tehran and Hormuzgan in southern Iran.
The people were being held in the custody of the revolutionary court, he said.
The US accuses Iran of seeking nuclear weapons.
Iran denies this, saying that its nuclear development programme is purely for peaceful, energy-generating purposes.