Russia says it is deploying a mobile version of its most important long-range nuclear missile.
The system is said to give Russia nuclear parity with the US
Defence Minister Sergei Ivanov said the new Topol-M missiles would be able to penetrate a multi-layered missile defence system.
Russia already has 42 fixed-site Topol-M missile systems, Russia's Itar-Tass news agency reports.
The missile, known in the West as the SS-27, has a range of more than 10,000km (6,200 miles).
Mounted on a heavy off-road launch vehicle, it is harder to detect than the earlier version, which has been in service for more than 20 years.
"These systems will form the basis of our strategic missile troops in the future. The first regiment is now being put on combat duty," Mr Ivanov said.
Under disarmament treaties, Russia and the US are committed to cutting their nuclear warheads to between 1,700 and 2,200 each.
Russian military expert Alexander Golts said the new Topol-M system would give Russia strategic nuclear parity with the US.