Iran's Bushehr nuclear power station has begun operating at a low level in a crucial step toward bringing it online, the Russian company that built the plant said Tuesday.

The generating unit at Iran's first atomic power plant was brought up to the "minimum controllable level of power" Sunday, Atomstroiexport, the state company that builds nuclear plants abroad, said in a statement.

"This means that a nuclear reaction has begun," said Vladislav Bochkov, spokesman for Atomstroiexport's parent company Rosatom. "This is one of the final stages in the physical launch of the reactor."

Begun in the 1970s by a German consortium, construction on the plant was abandoned after Iran's 1979 Islamic Revolution and has faced repeated delays since the mid-1990s, when Moscow reached a $1 billion deal with Tehran to complete it.

The United States and other Western nations for years urged Russia to abandon the project, fearing it would help Iran develop nuclear weapons. But an agreement obliging Tehran to repatriate spent nuclear fuel to Russia eased those concerns.

Still, Iranian politicians have blamed Russia for delays in the past.

Analysts say Russia, which has ties with Tehran but is one of the global powers seeking to ensure it does not develop nuclear weapons, has used the project as a lever in diplomacy.

On Monday, a member of an Iranian parliamentary commission monitoring Bushehr said "final tests" were being conducted, and Iran's Fars news agency said the plant would start providing power to the national grid within two months.

Bochkov said the reactor's operational and safety systems were being tested at the low power level. This will be increased gradually and brought to full capacity, "and after that it will be integrated into Iran's power grid," he said.

Bochkov gave no time frame for that.

Iran began loading fuel into Bushehr last August in front of foreign and domestic media, touting it as a symbol of resistance to international sanctions imposed by countries that suspect the Islamic state is seeking nuclear weapons, something it denies.

The plant had been expected to go online early this year, but ran into more delays including what officials said was a technical problem that required removal of the fuel assemblies.

The Bushehr plant's single reactor is to produce 1,000 megawatts, about 2.5 percent of Iran's electricity usage.

The oil-producing country says it plans a network of nuclear power stations.

It says it wants nuclear energy solely for peaceful purposes and has defiantly rejected international calls for a halt to uranium enrichment at its Natanz nuclear complex.