Canada's ambassador to the U.S. says the Nexus trusted-traveller program is being "held hostage" by an American effort to renegotiate the agreement.
Kirsten Hillman made the comments at a summit on border issues at the Canadian Embassy in Washington.
She says the U.S. is trying to renegotiate the terms of the Nexus agreement unilaterally.
"There's an attempt to renegotiate the terms of a 20-year-old program unilaterally and the program is being held hostage to that effort. The backlog is growing and growing every day, it's disappointing and frustrating for us."
The 13 Nexus enrolment centres in Canada remain closed due to a dispute over legal protections for the American pre-clearance officers who work there.