NATO allies cannot agree to Russian demands to limit their missile defenses because of the threat posed by North Korea, a senior U.S. State Department official said on Tuesday.
North Korea's claim last week to have tested a hydrogen bomb, which would represent an advance in its capability to strike Japan and the United States, has underscored Washington's determination to enhance the defenses that Russia opposes.
"We are not going to agree to limitations on our systems because we need to have the flexibility to deal with the dynamic and evolving threat," Frank Rose, deputy assistant secretary of state for arms control, told reporters at NATO in Brussels.
While there is considerable doubt over the veracity of Pyongyang's assertion that last week's explosion was a full-fledged test of a hydrogen device, Washington already warned last February that North Korea is seeking a long-range, nuclear-armed ballistic missile capable of reaching the United States.
NATO Secretary-General Jens Stoltenberg last week called on North Korea to end its nuclear and ballistic missile programs.
NATO's ballistic missile defense, in place since 2010, has been a source of tension between Russia and the U.S.-led alliance even before Russia's annexation of Crimea in 2014, although NATO says it is not designed against Moscow.
Russia threatened last year to aim nuclear missiles at Danish warships if Denmark joins NATO's missile defense system, arguing that it could reduce the effectiveness of its own nuclear arsenal.
"The key Russian concern ... is that in the future, absent legally binding constraints, we will develop systems that could potentially negate their strategic deterrent," Rose said.
While the United States provides much of NATO's missile shield, the alliance in 2012 agreed to develop its capabilities in Europe.
Romania has agreed to host a defense system, while Turkey already has a missile defense radar in place. The United States sent a destroyer to Spain in September, one of four ships that make up part of the shield.
Poland is also due to host defenses from 2018, with construction on a site starting this year.
"These capabilities are designed to defend NATO Europe against threats from outside the Euro-Atlantic area. They are not directed against Russia," Rose said.
Rose said Russian officials had voiced concern about the kind of technology that the United States did not have, such as a sea-based missile defense interceptor capable of speeds of 10 kilometers (6.21 miles) a second.