Despite nuclear saber-rattling by Moscow, U.S. nuclear forces are close to reaching warhead, missile, and bomber numerical cuts required under the 2010 New START arms treaty, a senior Pentagon official told Congress on Wednesday.

U.S. nuclear warheads counted under the treaty with Russia were reduced from 1,642 to 1,597 between Sept. 1 and March 1, said Robert Scher, assistant secretary of defense for strategy, plans, and capabilities.

Russian nuclear warheads were cut from 1,632 to 1,582 during the same period, Scher told the House Armed Services strategic forces subcommittee in prepared testimony.

The treaty requires both nations to reduce their deployed strategic warhead arsenals to 1,550 by February 2018.

For land-based and sea-based missiles and bombers, U.S. forces were reduced from 912 to 898, and Russian missiles and bombers were cut from 911 to 890.

The treaty limit for those weapons is 800 strategic delivery vehicles by 2018.

Scher said the reductions continued despite "serious concerns" over souring U.S.-Russia relations.