WASHINGTON  The Obama administration is increasingly emphasizing the idea that the United States will have forces in Afghanistan until at least the end of 2014, a change in tone aimed at persuading the Afghans and the Taliban that there will be no significant American troop withdrawals next summer.

In a move away from President Obama’s deadline of July 2011 for the start of an American drawdown from Afghanistan, Defense Secretary Robert M. Gates, Secretary of State Hillary Rodham Clinton and Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, all cited 2014 this week as the key date for handing over the defense of Afghanistan to the Afghans themselves. Implicit in their message, delivered at a security and diplomatic conference in Australia, was that the United States would be fighting the Taliban in Afghanistan for at least four more years.

Administration officials said the three had made loosely coordinated comments at the conference, in Melbourne, to try to convince Afghans that the United States was not walking away next summer and to warn the Taliban that aggressive operations against them would continue. Although Mr. Obama and administration officials have repeatedly said that July 2011 would be only the start of troop withdrawals, the Taliban have successfully promoted the deadline among the Afghan populace as a large-scale exit of the 100,000 United States troops now in the country.