Russia will keep observing the missile and warhead limits in the New START nuclear treaty with the United States, which expired last week, as long as Washington continues to do the same, Foreign Minister Sergei Lavrov said on Wednesday.

The 2010 treaty ran out on Feb. 5, leaving the world's two biggest nuclear-armed powers with no binding constraints on their strategic nuclear arsenals for the first time in more than half a century.

President Donald Trump declined a formal proposal from Russian President Vladimir Putin to voluntarily abide by the New START limits for another year. Lavrov said Moscow would stick to the limits itself for now anyway.

"Our position is that this moratorium on our side that was declared by the president is still in place, but only as long as the United States doesn't exceed the said limits," Lavrov told parliament's lower house, the State Duma.

The treaty's expiry has spurred fears of a three-way arms race involving Russia, the U.S. and China, which has far fewer warheads than the other two countries but is arming rapidly.

Some analysts say, however, that Russia is keen to avoid the cost of such a contest at a time when its state budget is feeling the strain from its four-year-old war in Ukraine.