MOSCOW (AFP) - Russian President Vladimir Putin on Friday signed into law controversial parliamentary legislation banning the adoption of Russian children by American families, the Kremlin said.The United States on Friday expressed deep regret after Putin signed the law, and said it hoped cases already under way would not be affected.“We deeply regret Russia’s passage of a law ending inter-country adoptions between the United States and Russia,” State Department spokesman Patrick Ventrell said in a statement, decrying the move as “politically motivated”.The law - retaliation for a US law punishing Russian officials implicated in the 2009 prison death of the whistle-blowing attorney Sergei Magnitsky - will come into force on January 1, it said.The highly contentious law is seen as the toughest piece of anti-US legislation during Putin’s 13-year rule and has prompted objections not just from activists but even some cabinet ministers.A particular subject of concern are the dozens of Russian children who are now in an agonising limbo after being prepared for US adoption but who now are effectively banned from leaving for their new lives.The Kremlin children’s rights envoy Pavel Astakhov said a total of 52 Russian children who had been prepared for US adoption should now go to Russian parents specially selected by regional governors.“I believe they should be adopted in Russia,” Astakhov, a key backer of the legislation, told Interfax.The law had been passed in three readings by the State Duma, the lower house of parliament, and then at a session Wednesday held by the Federation Council or upper house. Putin said on Thursday that he intended to sign it, ending days of speculation about his position.  Pro-Kremlin lawmakers put together the bill in a matter of days in response to a new US law sanctioning Russian officials implicated in Magnitsky’s death.Its passage revealed cracks in the Russian leadership between those who would prefer to get tough with the United States and ministers who back a more cautious approach.Foreign Minister Sergei Lavrov and the cabinet’s social affairs chief, Deputy Prime Minister Olga Golodets, have both spoken out against it.By apparent coincidence, a Moscow court on Friday also acquitted the former deputy chief of a Moscow prison where Magnitsky was held over causing his death by negligence.Dmitry Kratov was the only Russian official to still be facing charges over the Magnitsky case.