MOSCOW (AP) — Russian authorities have added a prominent independent election monitoring group to its registry of “foreign agents,” a move that is part of a relentless government crackdown on independent media and activists ahead of the September parliamentary election.

It is the second time that Golos, Russia's leading election watchdog founded in 2000, was slapped with the “foreign agent” designation, which implies additional government scrutiny and carries strong pejorative connotations that can discredit the recipient.

The group was first labeled as “foreign agent” in 2013 and liquidated as a non-governmental organization three years later. It has continued to operate without registering as an NGO, exposing violations at various elections.

On Wednesday Golos was added to a new registry of “foreign agents,” created by Russia's Justice Ministry for groups that are not registered as a legal entity in Russia. It is the only entry in the registry so far, however, a number of Russian NGOs in recent years switched to operating as informal entities in order to not be affected by the “foreign agent” designation.

Independent media, journalists, opposition supporters and human rights activists in Russia have faced increased pressure ahead of the Sept. 19 vote, which is widely seen as an important part of President Vladimir Putin’s efforts to cement his rule before the next presidential election in 2024.

The 68-year-old Russian leader, who has been in power for more than two decades, pushed through constitutional changes last year that would potentially allow him to hold onto power until 2036.

Co-chair of Golos Grigory Melkonyants called the move against the group on Wednesday “a serious signal about impending rigging.”

“We have no doubt that this attack on the largest community of independent election observers just a month before the voting day is an attempt to prevent Russian citizens from exercising the right the Constitutional Court sees as a guarantee of public recognition of election results,” Melkonyants said in a Facebook statement, adding that the group plans to continue its work regardless.