The United Nations has said around 5,000 people, including more than 2,000 civilians, were killed in fighting in Afghanistan last year alone.
There are currently around 70,000 foreign soldiers deployed in Afghanistan, and the United States will send an additional 17,000 soldiers over the next year.
Germany, the third largest troop contributing nation in Afghanistan with around 3,500 soldiers, has committed to send an extra 600 troops for the election.
The vote, according to the constitution to be held in spring, has been postponed until August 20 due to security concerns and logistical problems.
Karzai's term ends on May 21, but he has still not announced if he will step down then. His opponents have called for a caretaker administration to rule the country until the election.
Scheffer said this was an Afghan matter and the country's international allies should not interfere.
The spiralling violence in Afghanistan is expected to be a major focus of a Nato summit to be held on April 3.