"We are going to focus our military mission principally on training security forces," Merkel said in her weekly Internet blog.
"Germany is chiefly engaged in northern Afghanistan, and that is where we will fulfil our training commitment, in a speedier and more concentrated manner than hitherto," she said.
Merkel said she would meet Afghan President Hamid Karzai in Berlin on Tuesday and Wednesday to "discuss how best to coordinate Afghan and international efforts."
The chancellor was to address parliament on Wednesday on the government's policy on Afghanistan.
Berlin's participation in the 110,000-strong international force fighting the Taliban insurgency in Afghanistan, the third largest behind the United States and Britain, is deeply unpopular among Germans.
Merkel made no mention Saturday of sending more German troops to Afghanistan, having previously said that she would take no decision on this score before the London conference on Thursday.
But Defence Minister Karl-Theodor zu Guttenberg told the Frankfurter Allgemeine Zeitung newspaper that he would present before London a "concrete figure for an eventual increase of the participation of German troops."
The increase would however depend on the outcome of the London meeting, he
said in an interview to appear Monday.
Foreign Minister Guido Westerwelle, who would represent Berlin at the conference, also did not rule out boosting Germany's 4,300-strong force in remarks to be published Sunday.
"I've never said that we would send no extra troops, for example to train Afghan forces," he told the Bild am Sonntag weekly. "But I'm not giving a blank cheque."
Westerwelle also called for "using the current contingent in the best way."
Outgoing Afghan Foreign Minister Rangin Dadfar Spanta told the daily Tagesspiegel that "the training of troops and police must be strengthened."
"I haven't come to Europe to ask for more soldiers," he added. "We aim to take full responsibility in every province in 2015, so that our allies serving in Afghanistan can go home."
Polish Foreign Minister Radoslaw Sikorski said it an interview due to appear Sunday that Germany had no choice but to step up its troop presence in Afghanistan.
"It is not possible for a country not to want to engage itself basically for political or historical restraints," he told the Welt am Sonntag newspaper, referring to Germany's reticence to send troops abroad after World War II.
"We have to leave all that behind us," he said, adding: "We have to reinforce our efforts in Afghanistan now before withdrawing as soon as possible."