Stephen Johnson, the new US assistant defense secretary for the Western Hemisphere, also said Washington must not waver in its support for Colombia in the fight against guerrillas and drug traffickers.
His comments came as US congressional Democrats show reluctance to approve a trade deal and extend amultimillion-dollar aid package to Colombia due to concerns about the human rights record of President Alvaro Uribe.
''Right now funds for security assistance are slim and what programs we can offer are limited by complicated sanctions.
That leaves a vacuum for powers like China and Russia to fill,'' Johnson said in an interview posted yesterday on the Web site of the Miami-based Southern Command, which oversees US military operations in Latin America and the Caribbean.
The United States must take advantage of a ''pretty good'' situation in the region and do more to engage countries, Johnson said.
On Colombia, one of the United States' biggest military commitments outside the West Asia, he said Uribe's government had great success in pushing back guerrillas and drugs traffickers and improving the economy.
''Yet, all of a sudden, our Congress has gotten a case of Colombia-fatigue ... a few appear not to want it to succeed because they oppose the Administration.'' Uribe is Washington's closest ally in Latin America but is struggling to convince U.S. Democrats who control Congress that he has curbed suspected ties between some of his political allies and illegal paramilitaries accused of atrocities.
Johnson said the prosecutions of those cases should be seen as a positive result of justice reforms, not as setback.
''Now is not the time to quit on Colombia,'' he said.
Latin American armed forces have traditionally had close ties to the US military, but some nations have been reluctant to get involved in the US war on terrorism and say combating drug trafficking and organized crime is best left to police.
Johnson said Washington needed to do a better job of listening to the interests of partner nations. Development and institution building had to be part of US security policy.
''An approach informed solely by security considerations would be narrow and would certainly conflict with political and economic goals,'' he said.
Some critics have expressed concern that the US military is undermining civilian governments by pushing for social problems to be treated as security threats and taking on social projects better handled by civilian agencies.
REUTERS KK PM0425