Clinton will meet with President Hamid Karzai and other Afghan officials at a conference in Bonn that is also aimed at ensuring international financial and technical support continues after the troops withdraw in 2014.
The chief US diplomat will also meet her German and other counterparts from 100 countries and international organisations, but not from Afghanistan's key neighbour Pakistan, which announced a boycott after a deadly NATO bombing raid.
US officials have stopped short of offering an apology pending an investigation into the November 26 raid that killed 24 Pakistani troops on the troubled border.
Clinton has voiced regret over the boycott because she said Pakistan has a stake in a secure and stable Afghanistan but aides travelling with her denied Pakistan's absence would undermine the conference in this west German city.
Analysts have said the absence undermines efforts to promote peace talks between the Afghan government and the Taliban but US officials said neither Washington nor Kabul had current high expectations for reconciliation.
Pakistan is seen as having key influence over the Taliban.
"I don't think it (the boycott) will impact the conclusions of the conference in any way," a senior State Department official told reporters on the condition of anonymity during the flight to Bonn from Washington yesterday.