Nigerian soldiers on Sunday sealed roads heading out of a key northeastern city, blocking supply routes to remote towns where Boko Haram Islamists have taken power as a massive offensive against the insurgents spreads across the region.

The campaign against Boko Haram, which has said it wants to create an Islamic state in northern Nigeria, is aimed at re-establishing Nigeria's "territorial integrity," the military said after a state of emergency was declared in three areas.

The assault has included air strikes on Boko Haram strongholds in remote parts of northeastern Borno state, and has spread to the state capital Maiduguri, the insurgents' traditional home base - which residents said Sunday had been blockaded.

"There is a huge build-up of trucks loaded with essential commodities ... along the Baga road on the way out of Maiduguri to the northern part of the state," said resident Ibrahim Yahaya.

The phone network in Borno has all but collapsed since President Goodluck Jonathan on Tuesday declared a state of emergency there and in two neighboring states, Adamawa and Yobe.

Defense spokesman Chris Olukolade said in a statement Saturday that Islamist fighters who are on the run amid the offensive are scrambling for key supplies like fuel. He urged residents to report anyone trying to buy large quantities of fuel.

The supply shortage has begun to bite in the town of Gomboru Ngala.

The town has received an influx of people fleeing the nearby Marte district, one of the areas where Boko Haram chased out the government.