A security alert over a suspected bomb has shut down the main motorway through Belfast, triggering traffic chaos on the 20th anniversary of the Irish Republican Army's cease-fire.

Monday's closure of the M1 motorway forced morning commuters down gridlocked side streets. The disruption continued for several hours as British Army experts using a remote-controlled robot inspected the suspected bomb on a residential street beside the three-lane motorway.

The disruption came two decades after the outlawed Provisional IRA called an open-ended truce following a failed 24-year campaign to force Northern Ireland out of the United Kingdom. The Provisionals abandoned their truce in 1996, restored it a year later, and disarmed and formally renounced violence in 2005.

IRA splinter groups opposed to Northern Ireland's Catholic-Protestant government still mount occasional bombings and shootings.