Karzai has so far refused to sign the agreement setting out rules for Western training and security support after most combat forces pull out, frustrating Washington which has warned it may withdraw altogether and freeze its aid.
"We are urgently waiting for a signature from the Afghan side," said German Defence Minister Thomas de Maizière, whose country has offered to also keep hundreds of forces in the country post-2014.
"We are prepared for all options, but not endlessly," the minister told reporters on the flight to Afghanistan. "Such a deployment and redeployment require forward planning."
Karzai had first endorsed the deal -- which lays out rules for US troops, and would be the basis for other NATO forces -- but later said it could only be signed after an April presidential election.
De Maizière conceded that "public pressure" was unlikely to have an effect on Karzai, but warned that there was no time to lose in planning the complex operation.
"An agreement after the presidential election, which could go into a second round, and which would then be followed by a decision by the Americans, would certainly be too late," he said.
De Maizière also thanked German troops who have been deployed in the country's north in their longest combat operation since World War II, saying that in Afghanistan "German soldiers had to learn to fight".
The day the minister visited the northern city of Mazar-i-Sharif, a suicide bomber at Kabul airport detonated an explosives-laden vehicle near a German NATO military convoy, killing himself but causing no other casualties, officials said.