The New START deal, the centerpiece of Barack Obama's efforts to reset ties with Russia and the most significant arms control pact in nearly two decades, took effect last week. It limits each country to 1,550 strategic warheads, down from the current ceiling of 2,200.
The treaty doesn't prevent the U.S. from building new missile defense systems, but Russia has warned that it reserves the right to withdraw from the treaty if the United States significantly boosts its missile shield.
Russia's Deputy Foreign Minister Sergei Ryabkov reaffirmed Monday that a buildup in the U.S. missile defense capability would prompt Moscow re-consider its obligations under the New START treaty.
"If the U.S. increases the qualitative and quantitative potential of its missile defense ... a question will arise whether Russia should further abide by the treaty or would have to take other measures to respond to the situation, including military-technical measures," Ryabkov said, according to Russian news agencies.
Russia was strongly critical of the previous U.S. administration's plan to deploy missile defense sites in Poland and the Czech Republic and hailed Obama's decision to scrap it. But the Kremlin has remained concerned about revamped U.S. missile defense plans and continued to see them as potentially dangerous to its security.
NATO last fall approved a plan for a U.S.-led missile shield in Europe and invited Russia to join, but Moscow hasn't yet made a definite commitment. Experts from both sides will analyze the issue and report to defense ministers in July.
Ryabkov warned Monday that Russia won't cooperate with NATO on the project unless it's treated as a full partner.
"This must be a joint system with shared responsibilities, information exchange and decision-making in order to make us an equal and responsible member," he said. "If two separate networks are built, things won't change for us and we will see a situation when the NATO system could potentially be used against Russia's security interests. Cooperating on such a system would mean hurting ourselves."
Russian officials have remained skeptical about U.S. and NATO claims that the proposed shield is designed to fend off a missile threat from Iran.
Russia's Deputy Defense Minister Anatoly Antonov said Monday the U.S. missile defense could only be aimed against Russian missiles. "This system could undermine Russian nuclear deterrent forces," he said.