Meeting the 29 April 2004 international deadline for destroying 45% of its chemical WMD arsenal will take an additional three years, according to the Defence Department.
"The United States is therefore requesting the Organization for the Prohibition of Chemical Weapons (OPCW) grant an extension of the 45% destruction deadline," the Defence Department statement said.
The Pentagon expects the military to be ready to reach the required milestone by December 2007, the Pentagon said.
No detailed explanation for the postponement was given.
But the department pointed out that its chemical programme "has had several delays due to unresolved political and operational issues that forced operational shutdowns or postponed start-up dates."
Chemical WMD stockpile
1985, the Congress passed a law directing the US Army to destroy its obsolete chemical agents and munitions.
1992, the Congress tells US Army to plan for the disposal of materiel not included in obsolete stockpile.
US Army establishes the Nonstockpile Chemical Material Programme to dispose of the materiel.
In 1993, the United States signed the UN-sponsored Chemical Weapons Convention.
In October 1996, the US becomes 65th nation to ratify the convention making the treaty effective on 29 April 1997.
The United States stockpile of lethal chemical warfare munitions consists of various rockets, projectiles, and mines.
These contain blister agents, various forms of mustard gas and both VX and GB nerve agents, according to Virginia-based GlobalSecurity.org.
At the beginning of 2003, there was a stockpile of just over 110,000 tons of chemical weapons, stored at eight sites in the continental US and one site, the Johnston Atoll, in the Pacific.
About 60% of this stockpile is in bulk storage containers - 40% is stored in munitions, many of which are now obsolete.
Through ratification, the United States agreed to dispose of miscellaneous chemical warfare materiel by 29 April 2004.
But the US also agreed to destroy its unitary chemical weapons stockpile, binary chemical weapons, recovered chemical weapons, and former chemical weapon production facilities by 29 April 2007.