Given other troop movements, the extension means the Pentagon will have about 2,500 more soldiers in Afghanistan than previously planned, for the next few months.
Rice, meanwhile, told reporters she would ask congress for $8.6 billion in new money to train and equip the Afghan army and police and $2 billion for reconstruction projects.
"The challenges of the last several months have demonstrated that we want to, and should, redouble our efforts," Rice said as she flew to Brussels for a meeting of Nato foreign ministers on Friday that will focus on Afghanistan.
In 2006 more than 4,000 people died in fighting in Afghanistan and violence in the impoverished central Asian country is increasing.
Last year was the bloodiest in Afghanistan since US-led forces removed the Taliban from power in 2001 follwiong the September 11 attacks on New York and Washington.
The increase in forces comes as George Bush, the US president, is planning to send 21,500 extra troops to Iraq to combat growing sectarian violence.
Critics of Bush's policy in Iraq, including Hillary Clinton, a Democratic presidential hopeful, have urged Bush to focus instead on bolstering forces in Afghanistan.
At the Nato meeting on Friday Rice is expected to use the increased US deployment in Afghanistan to encourage other nations to commit further troops and funds to help combat the Taliban.
US army commanders have said they expect the Taliban to try to increase the level of violence again in spring, when fighting in Afghanistan traditionally picks up after a winter lull.