Nigerian troops backed by air strikes recaptured the northeastern towns of Monguno and Marte from Islamist Boko Haram insurgents on Monday, the military said in a statement.

Boko Haram fighters had seized Monguno on the shores of Lake Chad last month in an offensive that also targeted a town called Konduga and the outskirts and the airport of the main northeastern city of Maiduguri.

The conflict with Boko Haram has intensified in the past year and is a major issue in campaigning for a presidential election, now set for March 28, that pits President Goodluck Jonathan against former military ruler Muhammadu Buhari.

"The air and land operation is continuing with aggressive advance towards other designated communities and locations meant to be cleared in the ongoing offensive against the terrorists," defense spokesman Major-General Chris Olukolade said.

The militants, who are fighting to establish an Islamic state in religiously mixed Nigeria, have killed many thousands and are seen as the gravest security threat to Africa's top oil producer and biggest economy.

They also increasingly a source of instability to the entire region. Lake Chad is where Niger, Chad and Cameroon border with Nigeria, and all three neighbors have been sucked into the conflict with Boko Haram.

More than 5,000 residents fled when Monguno was attacked.

Nigeria last weekend postponed the national election set for Feb. 14 for six weeks, citing the security threat from Boko Haram, in a move that rattled markets and foreign investors.