Howland told reporters at the International Press Association of Colombia the U.N. welcomed the peace process but warned the talks could be difficult.
Colombian government representatives will meet with representatives of the Revolutionary Armed Forces of Colombia, known by its Spanish-language initials of FARC, in Oslo beginning Oct. 8.
Santos has ruled out conciliatory gestures or end to government operations against FARC guerrillas before the talks begin.
There won't be a cease-fire during the negotiations either, Santos said.
"I have asked that military operations be intensified, that there will be no cease-fire of any kind," Santos said. "We won't cede anything at all until we reach the final agreement. That should be very clear."
Howland was in Bogota to set out U.N. ideas promoting "transitional justice," which the world body says is the key to ensuring lasting peace in Colombia.
The United Nations wants the peace talks to result in measures that address human rights abuses and closure on more than 15,000 cases of forced disappearance recorded over the past three decades of the conflict.
FARC has been fighting successive governments in Colombia since 1964, making it the oldest insurgent group in South or Central America. Santos indicated he wants to include Colombia's second-largest rebel group, the National Liberation Army, in the peace talks.
If the talks work, Howland said, "there will be a deep transformation of the Colombian society and thus the emphasis that there should not be an amnesty for all crimes committed."
U.N. officials have said they hope the talks will lead to a process that produces "mechanisms to establish what happened and also compensations for the victims, individually or collectively."
Howland said, "It is quite clear in the international legal framework that an amnesty or a pardon cannot be included but it's not clear how far you must go with criminal law," indicating U.N. support for mechanisms that will help establish lasting peace.
Families of the disappeared want to know the fate of their loved ones and if possible receive information on their remains.
A DNA data base may be necessary to produce results from exhumations and information likely to be received from the rebels in the event of an accord. U.N. sources say.
"Families are suffering every day, not only from a moral point of view but also legally," the U.N. envoy said. "And until they don't know where their loved ones rest, human rights violations persist."
FARC leaders have said they want amnesty and assurances that those directly implicated in narcotics smuggling to North America won't be extradited to the United States.