Russia said Wednesday it was suspending joint research on nuclear energy projects with the United States, as Moscow's standoff with the West shows no sign of abating.
The Russian foreign ministry said Moscow could "no longer trust Washington in such a sensitive sphere".
The move came after Russian President Vladimir Putin on Monday ordered a halt to an agreement with the US on weapons-grade plutonium disposal in retaliation for Washington's "unfriendly actions".
Tensions have surged between the two powers after Washington suspended talks on a ceasefire in Syria over Moscow's continuing bombing campaign in support of President Bashar al-Assad's regime.
On Wednesday, the Russian government published orders signed by Prime Minister Dmitry Medvedev and dated Tuesday calling for the "suspension" of a 2013 agreement on cooperation in scientific research and development in the nuclear and energy spheres.
Russia said this was a response to the US regularly extending sanctions over Ukraine, including by restricting cooperation on nuclear energy.
The foreign ministry said in a statement that the US had already "practically frozen coooperation" and that "in this situation, the Russian side is suspending the fulfilment of agreement in answer to the unfriendly actions of the US."
The government said that in the current situation it was inadvisable to "allow American citizens into Russian nuclear installations" as well as to permit direct cooperation between Russian and US research institutions.
The suspension included an agreement between Russia's nuclear agency Rosatom and the US Department of Energy to study converting Russian research reactors to use low-enriched uranium, Moscow said.
The foreign ministry in its statement said that "we can no longer trust Washington in such a sensitive sphere as modernisation and ensuring the safety of Russian nuclear installations".
If Russia decides to convert research reactors to use low-enriched uranium, "we will carry out this work by ourselves," the ministry said.
In fact, cooperation "has been dormant for two years," a spokesman for Russia's nuclear agency Rosatom told TASS state news agency, with no new contracts planned.