Russian President Vladimir Putin has signed a law banning Americans from adopting Russian children, the Kremlin has said.

The law, which also imposes other measures in retaliation for US legislation designed to punish Russians accused of human rights abuses, will come into force on 1 January 2013.

It is likely to add to strains in relations with Washington.

Meanwhile, a Moscow court has acquitted the only official charged with the death of whistleblowing lawyer Sergei Magnitsky.

Mr Magnitsky died in jail in 2009 after his pancreatitis went untreated.

An investigation by Russia's presidential council on human rights concluded that Mr Magnitsky was severely beaten and denied medical treatment.

The Tverskoy court found jail doctor Dmitry Kratov not guilty of negligence causing death.

The case has become a rallying point for human rights advocates in Russia and abroad.

Congress in December passed legislation calling for sanctions against officials deemed human rights violators.

Mr Magnitsky's family and associates described the trial as sham, saying authorities failed to prosecute officials responsible.