Russia has embarked on a multi-billion-dollar military modernization program and Russia's top general, Valery Gerasimov, said last week that support for Russia's strategic nuclear forces combined with improvements in conventional forces would ensure that the United States and NATO did not gain military superiority.
He said the Russian military would receive more than 50 new intercontinental nuclear missiles this year.
In December, Putin signed a new military doctrine, naming NATO expansion as a key risk. Before the new doctrine was agreed, there had been some calls from the military to restore to the doctrine a line about the right to a first nuclear strike.
This was not included in the new doctrine, however, which says Russia reserves the right to use nuclear weapons in response to a nuclear strike or a conventional attack that endangered the state's existence.
NATO's 2010 "strategic concept" says deterrence, "based on an appropriate mix of nuclear and conventional capabilities, remains a core element of our overall strategy."
Washington and Moscow have traded accusations that the other has violated a Cold War-era arms control agreement.
The United States accuses Moscow of violating the 1987 Intermediate-Range Nuclear Forces treaty by testing a ground-launched cruise missile. Russia argues that Washington's use of drones and other intermediate-range arms amounts to a violation of the treaty.
A senior NATO official said Russia's Zapad exercise in 2013 was "supposed to be a counter-terrorism exercise but it involved the (simulated) use of nuclear weapons".
The Arms Control Association, a Washington-based advocacy group, estimates Russia has about 1,512 strategic, or long-range, nuclear warheads, a further 1,000 non-deployed strategic warheads and about 2,000 tactical nuclear warheads.
Tactical nuclear weapons can include short-range missiles and artillery shells, mines and bombs.
The United States had 4,804 nuclear warheads as of September 2013, including tactical, strategic, and non-deployed weapons, according to ACA.
Among other NATO allies, France has fewer than 300 operational nuclear warheads and Britain has fewer than 160 deployed strategic warheads.