TEHRAN — Iran began building its second nuclear power plant with Russian help Saturday, the first such project since last year’s landmark nuclear deal with world powers.

The project in the southern port city of Bushehr will eventually include two power plants expected to go online in 10 years. Construction on the second plant is set to begin in 2018. The entire project will cost more than $8.5 billion, with each plant producing 1,057 megawatts of electricity.

‘‘Construction of the power plant is a symbol of Iran enjoying the results of the nuclear deal,’’ Senior Vice-President Ishaq Jahangiri said at a ceremony marking the start of the project.

‘‘We will continue working with Russia as a strategic partner and friend,’’ he added.

Iran’s sole operational nuclear reactor, also built in Bushehr with Russian assistance, produces 1,000 megawatts. It went online in 2011, and the two countries have agreed to cooperate on future projects.

Iran has a current capacity of 75,000 megawatts, nearly 90 percent coming from fossil fuels. It hopes to generate 20,000 megawatts of electricity through nuclear power in the next 15 years.

Sergey Kiriyenko, the head of Russia’s atomic agency, told reporters the plants would be built according to high safety standards, ‘‘particularly those defined after Fukushima,’’ referring to the Japanese nuclear power plant that was badly damaged in the 2011 earthquake and tsunami.

Iran’s top nuclear official, Ali Akbar Salehi, said the project would be monitored by the International Atomic Energy Agency and was in line with the Non-Proliferation Treaty.

He called the project ‘‘a new page in the trend of our peaceful industrial nuclear activities.’’

Western nations do not view the Bushehr plant as a proliferation risk because Russia supplies the fuel for the reactor and takes away spent fuel that could otherwise be used to make weapons-grade plutonium.

Russia, along with the United States, Britain, France, Germany, and China, reached a deal with Iran last year in which Tehran agreed to curb its nuclear program in exchange for the lifting of international sanctions. Iran rejects Western allegations that it is covertly seeking nuclear weapons, insisting its atomic program is for entirely peaceful purposes.

In a separate development, a British-Iranian employee of the Thomson Reuters Foundation, who was arrested more than five months ago while visiting family in Iran, was sentenced to a five-year prison term for unexplained charges.

Iran’s incarceration of the employee, Nazanin Zaghari-Ratcliffe, 37, and her secretive prosecution by a revolutionary court have created new tensions with Britain, just as the two countries have taken steps to improve their troubled relationship.

Last week, Iran and Britain announced they had agreed to raise diplomatic ties to the ambassador level for the first time since 2011, when relations were strained after an Iranian mob attacked the British Embassy in Tehran.

The Thomson Reuters Foundation, the independent charitable arm of the Thomson Reuters news agency, said in a statement Friday it was outraged by the sentence imposed on Zaghari-Ratcliffe, which was communicated to her family but was not publicly announced in Iran.