The death of a Russian child adopted in the United States will be a key topic when Foreign Minister Sergei Lavrov meets US counterpart John Kerry next week, a senior diplomat said Thursday.

"This issue, primarily the death of Maxim Kuzmin, will be among the key issues on the agenda of Lavrov's negotiations with Kerry scheduled for next week," Russian Foreign Ministry Human Rights Commissioner Konstantin Dolgov told a State Duma committee on family, women and children affairs.

He did not reveal when or where the two top diplomats would meet.

Claiming the US side was taking the case "under special control," Dolgov said Moscow would obtain all the necessary information. "They will do whatever they can so that the case is investigated properly," the Interfax news agency quoted him as saying.

Dolgov said numerous injuries, both external and internal, had been found on the boy's body during an autopsy. If Kuzmin's foster mother was proven to be the killer, "we'll seek the most severe punishment," the official said.

Acting US Consul in Russia Bill Bistransky told Kommersant's radio station in response the autopsy results had not been made public yet. "It's now too early to draw any conclusions on the causes of the child's death," he said.

Bistransky also noted Kirill Kuzmin, Maxim's brother, would stay with his adoptive father. "It is premature to discuss the boy's returning to Russia as he is a citizen of the US and lives in a specific space under the control of the local law enforcement and judicial bodies," he said.

Pavel Astakhov, Russian children's rights ombudsman, said in an interview with a local TV station the boy's foster mother would be prosecuted regardless of the case's development.

"Even if she is found not guilty of deliberate infliction of damage and manslaughter, leaving a child in danger is a crime in the state of Texas," Astakhov said, adding the mother left Kuzmin alone in the playground and had left him at home.

The woman, called L. Shatto by Russian authorities, has confessed to the investigators the boy lost consciousness while playing outside while she was in the house not long before the tragic incident.

US Ambassador to Russia Michael McFaul, in response to Alexei Pushkov, head of the State Duma committee on international affairs, wrote on his Twitter account he was open to discuss adoption matters, but would not attend hearings on the issue at the State Duma.

"As a norm, US Ambassadors do not participate in hearings of foreign parliaments. Do Russian ambassadors?" he wrote, addressing Pushkov, who said on Twitter earlier Thursday that, "by refusing to come to the State Duma...the US ambassador showed that they are not prepared for a serious dialogue on this problem."

Kuzmin, 3, was adopted from the same orphanage in the Pskov region where Dmitry Yakovlev was raised before being adopted by a US family and dying in 2008. A Russian law prohibiting adoptions of children by US citizens, which took effect on Jan. 1, was named after Yakovlev.