MOSCOW (Reuters) - Russian President Vladimir Putin has approved a law which will tighten controls on civil right groups funded from abroad, the his press office said on Saturday, a move opponents say is part of a campaign to suppress dissent.

The law, which was cleared by the upper house of parliament and the Federation Council earlier in July, will force non-governmental organizations (NGOs) engaging in "political activity" to register with the Justice Ministry as "foreign agents" and to file a report to officials every quarter.

Opposition groups say Putin is trying to silence groups whose criticism of his human rights record has undercut his credibility and helped fuel seven months of protests against his rule, the biggest since he came to power in 2000.

Putin, a former KGB spy, has been in power for 12 years as prime minister or president and he won another six-year stint in March.

Earlier in July, the U.S. State department expressed "deep concern" about the NGO law - and was promptly rebuked by Russia for "gross interference", an exchange that underlined the impact the bill has had on already strained relations.

(Writing by Andrey Ostroukh; Editing by Mark Heinrich)