MOSCOW — Russia threatened on Wednesday to deploy missiles to target the U.S. missile shield in Europe if Washington fails to assuage Moscow’s concerns about its plans, a harsh warning that reflected deep cracks in U.S.-Russian ties despite President Barack Obama’s efforts to “reset” relations with the Kremlin.

President Dmitry Medvedev said he still hopes for a deal with the U.S. on missile defense, but he strongly accused Washington and its NATO allies of ignoring Russia’s worries. He said Russia will have to take military countermeasures if the U.S. continues to build the shield without legal guarantees that it will not be aimed against Russia.

The U.S. has repeatedly assured Russia that its proposed missile defense system wouldn’t be directed against Russia’s nuclear forces, and it did that again Wednesday.

“I do think it’s worth reiterating that the European missile defense system that we’ve been working very hard on with our allies and with Russia over the last few years is not aimed at Russia,” said Capt. John Kirby, a Pentagon spokesman. “It is … designed to help deter and defeat the ballistic missile threat to Europe and to our allies from Iran.”

White House spokesman Tommy Vietor said the U.S. will continue to seek Moscow’s cooperation, but it must realize “that the missile defense systems planned for deployment in Europe do not and cannot threaten Russia’s strategic deterrent.”

Medvedev said Moscow won’t be satisfied by declarations and wants a binding agreement. He said, “When we propose to put in on paper in the form of precise and clear legal obligations, we hear a strong refusal.”

Medvedev warned that Russia will station missiles in its westernmost Kaliningrad region and other areas, if the U.S. continues its plans without offering firm and specific pledges that the shield isn’t directed at its nuclear forces. He didn’t say whether the missiles would carry conventional or nuclear warheads.

In Brussels, NATO Secretary-General Anders Fogh Rasmussen said he was “very disappointed” with Russia’s threat to deploy missiles near alliance nations, adding that “would be reminiscent of the past and … inconsistent with the strategic relations NATO and Russia have agreed they seek.”

“Cooperation, not confrontation, is the way ahead,” Rasmussen said in a statement.

The U.S. missile defense dispute has long tarnished ties between Moscow and Washington. The Obama administration has repeatedly said the shield is needed to fend off a potential threat from Iran, but Russia fears that it could erode the deterrent potential of its nuclear forces.