UPDATE 1-No signs of Qaeda at US-Mexican border-US official

(Adds militant tried to enter from Canada in third paragraph)

By Randall Mikkelsen

WASHINGTON, April 11 (Reuters) - Authorities have seen no signs of al Qaeda trying to insert operatives into the United States from Mexico, but the militant group has considered doing so, a U.S. intelligence official said on Friday.

The comments by Charles Allen, Homeland Security undersecretary for intelligence and analysis, could undercut one argument by advocates in and out of government for get-tough tactics to fight illegal crossings at the southern U.S. border -- that they are needed to fight terrorism.

In contrast, at least one Islamist militant has been caught trying to enter the United States from Canada by land to attempt an attack.

"We know of no trained al Qaeda operatives who have crossed over our southern border," Allen told reporters.

"We do know that going back to 2004, the southern border is something that al Qaeda's central leadership has looked at. But we know of no specifics of where al Qaeda has really endeavored to cross our borders in the south," he said.

The U.S. government is seeking to complete this year a planned 670-mile (1,080 km) border fence to fight illegal crossings from Mexico.

Homeland Security Secretary Michael Chertoff has cited the anti-terrorism argument in waiving environmental laws and other legal restrictions to quickly build the fence.