Sergey Lavrov said current US plans did not present a strategic threat
Russia's foreign minister has said his country could opt out of a new nuclear disarmament treaty if it feels threatened by US missile defence plans.
But Sergey Lavrov said current, short-term US plans, which could include ground-based interceptor missiles in Romania, seemed acceptable to Russia.
Russia and the US are due to sign a deal to reduce their nuclear stockpiles in Prague on Thursday.
Mr Lavrov said the treaty marked a "new level of trust" between the two states.
The treaty succeeds the 1991 Start treaty on nuclear disarmament, which expired in December.
It restricts both Russia and the US to 1,550 warheads, about 30% less than currently allowed.
The treaty, which was agreed last month, was delayed by Moscow's concerns over US missile shield plans.
US President Barack Obama scrapped a previous plan for a missile shield based in Poland and the Czech Republic that had angered Russia.
'Nuclear free world'
"We have noted that the announced plans proceed from the fact that at the first stages this system will not have strategic characteristics," Mr Lavrov said of current US plans.
NEW START TREATY
Russia (2010): 2,600
US (2009): 2,252
New Start: 1,550 each, maximum
Russia (2010): 566
US (2009): 798
New Start: 700 each, maximum
Source: White House and Bulletin of the Atomic Scientists
But he said Russia would have to "see what will happen next".
"Russia will have the right to abandon the Start treaty if a quantitative and qualitative build-up of the US strategic anti-missile potential begins to significantly affect the efficiency of Russia's strategic forces," he said.
The Russian foreign minister also offered qualified backing for the goal of a world free of nuclear weapons that President Obama set out in a speech in Prague last year.
"We call on every state without exception, but first and foremost those that possess nuclear arsenals, to join Russia and the US in their effort in this area and make their own active contribution to the process of disarmament," he said.
But he added that attention would first need to be paid to space-based weapons and missiles fitted with conventional warheads.
"To move toward a nuclear-free world, it is necessary to resolve the question of non-nuclear-equipped strategic offensive weapons and strategic weapons in general, which are being worked on by the United States, among others," he said.
"World states will hardly accept a situation in which nuclear weapons disappear but weapons that are no less destabilising emerge in the hands of certain members of the international community."