"The people behind this horrible act must be found and brought to justice and every form of support for the terrorists must be opposed," he said in a letter of condolence to his Pakistani counterpart Shah Mehmood Qureshi.
"We cannot say often enough that terror and violence must not be used as a tool in a political conflict," he added, condemning the attack "in the strongest terms."
There has been no claim of responsibility for the attack Saturday at the Marriott Hotel in Islamabad, frequented by foreigners, which killed 53 people and injured 266 people, according to a toll announced Sunday.
The Foreign Ministry in Berlin confirmed that seven Germans were injured in the attack, one severely. A spokeswoman said that the ministry could not confirm reports that one German national had died but said that all reports were being investigated.
Pakistani investigators have said the bombing was likely carried out by Taliban militants with links to Al-Qaeda from tribal areas near the border with Afghanistan.