President Donald Trump said Wednesday that the number of military troops deployed to the US-Mexican border could go as high as 15,000 as he draws a hard line on immigration in the lead-up to the midterm elections.

With his eyes squarely on next Tuesday's contests, Trump has rushed a series of immigration declarations, promises and actions as he tries to mobilize supporters to retain Republican control of Congress. His own Republican campaign in 2016 concentrated on border fears, and that's his focus in the final week of the midterm fight.

"As far as the caravan is concerned, our military is out," Trump said Wednesday. "We have about 5,800. We'll go up to anywhere between 10,000 and 15,000 military personnel on top of Border Patrol, ICE and everybody else at the border."

Trump rejected the idea he was "fearmongering" or using the issue for political purposes, but his escalating rhetoric in the waning days of the campaign season call into question the denial.

Trump has railed against illegal immigration, including several caravans of migrants from Central America slowly moving toward the US border. The caravan is still nearly 1,600 kilometres from the border.

He's also promised to end so-called catch-and-release policies by erecting tent cities to hold those crossing illegally. And this week he is asserting he could act by executive order to unilaterally end birthright citizenship for the children of non-US citizens.

Trump's comments on Wednesday appeared to catch the Pentagon off guard. The Pentagon on Monday directed 5,239 active-duty troops to deploy to the border to assist Customs and Border Protection agents in Texas, Arizona and California.

That is in addition to 2,092 National Guard troops who have been along the border for several months on a separate, but related, mission.

On Tuesday, Air Force Gen. Terrence O'Shaughnessy, commander of US Northern Command, which is supervising the new troop operation, disputed a news report that the active-duty total could reach 14,000.

"I honestly don't even know where that came from," he said. "That is not in line with what we've been planning."

O'Shaughnessy said the 5,239 number will increase, but he would not say by how much or when. Other officials have said that 2,000 to 3,000 additional active-duty troops are on standby for possible deployment to the border.

A deployment of 15,000 would bring the military commitment on the border to roughly the same level as in war-torn Afghanistan.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)