The United States believes last week's deadly explosion in Russia was associated with the Kremlin's hypersonic cruise missile programme, a senior Trump administration official said on Tuesday.

Five scientists were buried in the closed city of Sarov on Monday after what state nuclear agency Rosatom said was an accident during a rocket test on a sea platform off northern Russia.

The U.S. official, speaking on condition of anonymity, said Washington was not prepared to say at this point whether it was a nuclear explosion but believed it did involve radioactive elements.

The explosion could represent a potentially significant setback to the Russian programme although it remained unclear whether it was caused by a launching failure, the official told reporters.

Russian attempts to develop hypersonic cruise missiles raised questions about whether the so-called New Start nuclear agreement, which is due to expire in early 2021, should be extended for five years, the official added.

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