More often, the bombs are made from fertilizer, which has led the Afghan government to ban certain fertilizers in strategic regions such as southern Afghanistan.
The Pentagon figures do not include CIA, Drug Enforcement Administration or other civilians killed in the war, including the seven intelligence operatives killed in a base attack in late December 2009.
The Pentagon also keeps a separate tally of fallen service members and Defense Department civilians who have died supporting Operation Iraqi Freedom. 3,479 military service members and 9 department civilians have been killed in action in Operation Iraqi Freedom, according to Pentagon reports.
While the death toll in Afghanistan climbs, U.S. losses in Iraq continue to go down with the improved security situation.
As the United States prepares for a major drawdown of forces this year to 50,000 by September, the number of American forces in Afghanistan now exceeds the number in Iraq.
There are now 94,000 U.S. troops in Afghanistan and 92,000 serving in Iraq, as of May 24.
At least 313 U.S. soldiers died in Afghanistan last year, nearly twice as many as in 2008. In Iraq, 150 Americans lost their lives, half as many as the year before.
December 2009 was also the first month without a U.S. combat death in Iraq since the U.S.-led war there began in March 2003.