Iran began pouring concrete on Sunday for a second nuclear reactor at its Bushehr power plant after saying the day before that it had enriched uranium to five percent. The sequence of actions, analysts said, is thought to be in retaliation against a tightening of sanctions by the United States.

Teheran and Moscow inaugurated a new phase of construction for another reactor at Iran's sole nuclear power plant in Bushehr on the Gulf coast, a facility Teheran points to as its reason to break the enrichment limit set by its 2015 nuclear deal with world powers.

The reactor is one of two officially under construction since 2017 at the Bushehr site, around 750 kilometers south of Teheran.

The moves are the latest taken by Iran in defiance of the toughened sanctions from the US, said Niu Song, a professor at the Middle East Studies Institute of Shanghai International Studies University.

"The series of actions are a response to the US' increasing pressures, such as expanding sanctions to cover nine individuals who, as the US claimed, have close ties to Iran's Supreme Leader Ayatollah Ali Khamenei," he said.

Niu said Iran's actions are not meant to undermine the nuclear deal, but to safeguard the existence of the deal with extreme measures.

"Iran said it wants to maintain close interactions with the International Atomic Energy Agency, which will continue to carry out inspection work in Iran. When the right time comes, such as after some agreements are achieved, Iran's actions can be reversed."

On Nov 5, Iranian President Hassan Rouhani announced the country's fourth step to reduce its commitments to the nuclear deal, including its work to start injecting uranium hexafluoride gas at its Fordow reactor.

Despite the quickened pace of Iran's work in its capabilities for centrifuge technology and enriched uranium, Niu said it is hard to say that the activities are related to nuclear weapons research.

"Iran itself has the right to use nuclear energy in a peaceful way, and Europe will tend to continue its dialogues with Iran to mediate in the situation," Niu said.

The 2015 nuclear deal placed restrictions on the sort of nuclear reactor Teheran could develop and on its production of nuclear fuel. But it did not require Iran to halt its use of nuclear energy for power generation.

Niu said the US may step up its criticism of Iran and even extend sanctions to more Iranian high-ranking officials.

The US unilaterally withdrew from the agreement in May 2018.

AP contributed to this story.