Russia' s president Vladimir Putin (Reuters)

Russia's only independent polling agency, Levada Centre, said it could be forced to suspend all activities after it was targeted it under a controversial new law sponsored by president Vladimir Putin.

The centre published a letter from a Moscow prosecutor, dated last week, who claimed that the agency's polls were "aimed at shaping public opinion on government policy" and demanding it stopped all publication until it registered as a "foreign agent" under a law passed last year.

The law, which recalls Soviet-era anti-spy propaganda, requires all foreign-funded NGOs that engage in political activities to register as "foreign agents".

Levada Centre head Lev Gudkov said the organisation was in a difficult situation.

"In effect [this forces] it to cease its activity as an independent sociological research organisation, carrying out systematic polls of public opinion in Russia," he said.

He said that the prosecutor's action undermined its credibility and reputation. Gudkov claimed that foreign funding amounted to only 1.5-3% of the centre's budget each year.

The UN has condemned Russia over Putins' clampdown on NGOs. The Moscow offices of Amnesty International and Transparency International were among the hundreds searched by Russian inspectors in more than 50 regions of the country over the last months, as part of what Human Rights Watch described as a "crackdown on civil society".

The UN panel said the foreign agents law has "obstructive, intimidating and stigmatising effects" and breaches international law.