In a press release summarizing progress since 2009 on the policies known as the Prague Nuclear Agenda the Obama administration announced that it "unilaterally reduced the U.S. nuclear stockpile by approximately 500 warheads this year [apparently FY2016] and added these weapons to the dismantlement queue." Despite this reduction, no additional fissile material was declared as excess to weapon or military purposes.
The White House factsheet noted that "As of September 2016, the U.S. active stockpile of nuclear warheads consisted of 4,018 warheads." Previous data released by the United States showed that its active stockpile as of September 2015 included 4,571 weapons. The data indicates a reduction of 553 warheads.
As of September 2016, the warhead dismantlement queue has increased to about 2,800 from the "approximately 2,500 warheads retired and in the queue for elimination" as of September 2014 (announced in April 2015).
The press release reports that "from fiscal year 2009 through the end of fiscal year 2016, the U.S. dismantled 2,226 warheads and retired an additional 1,255 weapons." The average warhead dismantlement rate has fallen significantly in this period - the press release notes that "Since September 30, 2013, the United States has dismantled 666 nuclear warheads." This suggests that 1560 warheads were dismantled from 2009 to 2013 and only 666 warheads were dismantled since 2013. In the past, the United States was able to dismantle well over 1,000 warheads per year.
It appears that 258 warheads were dismantled in fiscal year 2016. At the current rate of dismantlement, it will take over a decade to dismantle the 2,800 warheads currently in the queue for elimination.
Among other achievements mentioned in the press release is the "removal of highly enriched uranium and plutonium from more than 50 facilities in 30 countries - more than four metric tons of nuclear material." The document notes that "since 2009, 16 nations and Taiwan - countries from Argentina and Libya to Serbia and Vietnam - have eliminated their holdings of highly enriched uranium and plutonium, making Latin America, Central Europe, and Southeast Asia completely free of these dangerous materials."