WASHINGTON (AFP) – A group of 25 US senators, almost all Democrats, released a letter urging President Barack Obama to carry out a "sizable and sustained reduction" of US forces in Afghanistan in July.

Obama promised 18 months ago -- when he announced a troop surge to push back the Taliban and buy time for Afghan political and security development -- that in July 2011 US troops "will begin to come home."

The senators, including a leftist political independent and two "Tea Party" conservative Republicans, said they believe that the US mission in Afghanistan is complete, especially after US commandos killed Al-Qaeda chief Osama bin Laden in early May.

Widespread corruption in Afghanistan has also undermined development goals, and staying there no longer justifies risking US lives or spending US funds.

"We write to express our strong support for a shift in strategy and the beginning of a sizable and sustained reduction of US military forces in Afghanistan, beginning in July 2011," wrote the senators.

US forces "rightfully and successfully intervened in Afghanistan" in 2001 to destroy a safe haven for the Al-Qaeda terror network, "removing the Taliban government that sheltered Al-Qaeda, and pursuing those who planned the September 11 attacks on the United States.

"Those original goals have been largely met," the letter read.

"Given our successes, it is the right moment to initiate a sizable and sustained reduction in forces, with the goal of steadily redeploying all regular combat troops," the letter read.

While building new civic institutions in Afghanistan "is a laudable objective," this goal "does not justify the loss of American lives or the investment of hundreds of billions of taxpayer dollars."

The United States "will never be able to secure and police every town and village in Afghanistan. Nor will we be able to build Afghanistan from the ground up into a Western-style democracy."

The cost of prolonging the war "far outweigh the benefits. It is time for the United States to shift course in Afghanistan.

"We urge you to follow through on the pledge you made to the American people to begin the redeployment of US forces from Afghanistan this summer, and to do so in a manner that is sizable and sustained, and includes combat troops as well as logistical and support forces."

There are some 100,000 US military personnel currently deployed in Afghanistan.

According to a recent CNN-Opinion Research poll, 30 percent of Americans favor a full withdrawal of US forces from Afghanistan.

Senators who signed the letter include Democrats Charles Schumer, Patrick Leahy, Barbara Boxer and Tom Harkin; Republicans Rand Paul and Mike Lee; and leftist independent Bernard Sanders.