US Secretary of State Hillary Clinton has sought to calm fears in Latin America about a planned new military agreement with Colombia.
Some countries in the region have expressed alarm over the US plans to use Colombian bases to combat drug traffickers and rebels.
But Mrs Clinton said the accord would respect Colombian sovereignty and other countries would not be affected.
It would not lead to a significant increase in US troop numbers, she said.
Speaking after talks in Washington with Colombian Foreign Minister Jaime Bermudez, Mrs Clinton said the agreement would not lead to the creation of US bases in Colombia.
"It does provide the United States access to Colombian bases but command and control, administration and security will be Colombia's responsibility," she said.
"Any US activity will have to be mutually agreed upon in advance. The United States does not have and does not seek bases inside Colombia."
She also said there would be "no significant permanent increase in the US military and contractor presence in Colombia" and that other countries would not be affected.
"This is about the bilateral co-operation between the United States and Colombia regarding security matters within Colombia," she said.
Under the deal, the US military will be able to operate on Colombian soil to tackle drug-trafficking and terrorism.
Mrs Clinton said the threats were very real and that the US was "committed to supporting the government of Colombia in its efforts to provide security to all its citizens".
Mr Bermudez said developing "more effective mechanisms of co-operation" would benefit both Colombia and the region.
"We have suffered, and we have learned from the lessons as a result of this suffering," he said.
A number of countries in the region have condemned the plan and Argentina has called the agreement "not helpful".
Venezuelan President Hugo Chavez has expressed fears the move would amount to preparation for an invasion of his country by US forces.