Buchanan also said his mission guidelines were clear - to support CBP personnel. He said his work was apolitical.
"I'm not being directed to do anything unnatural from above me," said Buchanan, who is commander of U.S. Army North.
The Pentagon says there are no plans for U.S. forces to interact with migrants and instead have been carrying out support tasks for CBP, like stringing up concertina wire and building temporary housing for themselves and CBP personnel.
In recent days, up to 1,000 migrants linked to the caravans have arrived in the Mexican border city of Tijuana, with a similar number expected to arrive in the next day or so. Thousands more could arrive in border towns over the coming days as the bulk of the caravans arrive.
Buchanan estimated that about 5,800 troops were deployed in total, with about 1,500 in California, 1,500 in Arizona and 2,800 in Texas. Buchanan acknowledged he might shift forces east or west along the border if needed.
Mattis told reporters earlier on Wednesday that U.S. soldiers were making rapid progress erecting barriers along the border and estimated the first, construction phase of the U.S. military effort could be completed within 10 days.
Buchanan suggested troops would go home once they had fulfilled requests by CBP.
"At some point in time, I'm not going to keep troops here just to keep them here. When the work is done, we’re going to start downsizing some capability," Buchanan said.
Buchanan would need to make any recommendations on redeployment of troops to General Terrence O'Shaughnessy, the head of U.S. Northern Command. O'Shaughnessy would then report to Mattis.
He suggested a recommendation could be made in the near future.
"I'm looking as early as next week to start thinking through rightsizing, if we need to change. Or do I need to shift (troops elsewhere on the border)," Buchanan said, without predicting when changes might occur.
