By Sonia Elks

MOSCOW (Reuters) - Russian lawmakers appealed to U.S. Congress on Friday to help return a Russian boy living with a Texas family, after the death of his three-year-old brother intensified a dispute over international adoptions.

The motion, approved overwhelmingly without any votes against by the lower house, is likely to increase tension between Moscow and Washington in a row that has contributed to a gradual cooling of relations since Vladimir Putin returned to the presidency last May.

Moscow has seized on the death of Max Shatto to justify a ban imposed on Americans adopting Russians on January 1, which was itself a response to U.S. legislation that imposes visa and asset bans on Russians accused of violating human rights.

Texas officials say Max Shatto was last seen alive on January 21, and U.S. authorities are trying to determine exactly what happened to him. His adoptive mother Laura Shatto, who is still looking after Max's two-year-old brother Kirill, said she found Max unresponsive in the backyard and he was taken to hospital where he died.

Russia has announced an inquiry into allegations he was beaten before his death but U.S. police and child welfare authorities only say the matter is part of a criminal investigation.

The parliamentary motion, using the brothers' original Russian surname, called on U.S. Congress to "support the Russian Federation in deciding the matter of returning Kirill Kuzmin back to the country of his origin for the sake of humanitarian reasons and the child's security".

The lawmakers, who did not specify how U.S. Congress could help in the case, also called on the United States to ensure the safety of adopted children and pass on any details of abuse of Russian-born children.

The boys' biological mother launched an emotional plea this week for Kirill to be sent back to Russia.

AMBASSADOR "TROUBLED"

The U.S. ambassador to Russia, Michael McFaul said he was "troubled" by responses in Russia to Max Shatto's death.

"It is time for sensational exploitations of human tragedy to end and for professional work between our two countries to grow, on this issue and many others," he wrote in a blog post.

Russia banned U.S. adoptions in retaliation for the U.S. Magnitsky Act, drawn up over concern about the death in prison of anti-corruption lawyer Sergei Magnitsky in 2009. The act will deny visas to Russians accused of human rights abuses and freeze their assets in the United States.

Until the ban, Americans adopted more Russian children than any other country, with more than 60,000 cases since the 1991 collapse of the Soviet Union, including 962 in 2011.

Russia considers more than 650,000 of its children to be orphans and nearly 129,000 awaiting adoption in late 2011.

Tens of thousands protested in Moscow in January against banning U.S. adoptions, some calling Putin a "child-killer".

Russia's children rights ombudsman Pavel Astakhov, an ally of Putin, told a news conference this week that foreign adoptions caused "colossal harm" to society.

But Putin's spokesman, Dmitry Peskov, said the Kremlin was against a full ban on all foreign adoptions.

Kremlin critics say the campaign against abuse of Russian-born children abroad is partly aimed at lifting Putin's approval ratings among conservative voters with nationalist views.

A February poll by the independent Levada Centre showed 47 percent of Russians considered the adoption ban to be a fitting response to the "Magnitsky Act".

(Reporting By Sonia Elks; Editing by Jason Webb)