Boko Haram Attack Threatens 'Hundreds of Thousands'

— -- Innocent civilians are said to be trapped as Boko Haram, the Islamist terror group, clashes with Nigerian forces near the major city of Maiduguri.

According to activists and locals, gunmen attacked a nearby military base in the early morning hours on Sunday. They also targeted the nearby town of Monguno. A fierce battle ensued, with the Nigerian military responding with air strikes and tanks.

"These ongoing attacks by Boko Haram are significant and grim news. We believe hundreds of thousands of civilians are now at grave risk," said Netsanet Belay, Amnesty International's Africa Director.

Maiduguri, home to more than 600,000 people, is one of Boko Haram's biggest targets to date. Many of the town's residents had fled Boko Haram's violence elsewhere in the country.

Locals said many residents on the outskirts of town had fled to the center for safety. Yet even there, safety is not guaranteed. Most residents are too afraid to venture into the streets unarmed. A curfew is in place and local security forces are also mobilizing to defend the town, they said.

The offensive comes as Secretary of State John Kerry arrived in Nigeria on Sunday ahead of next month's presidential election. He met Nigeria's President Goodluck Jonathan and his leading opponent, Muhammadu Buhari, Nigeria's former dictator who lost the last election in 2011.

Kerry cautioned each of them not to enflame tensions, warning that if either side promotes violence they will not be welcome in the United States.

"Anyone who participates in, plans, or calls for widespread or systematic violence against the civilian population must be held accountable, including by ineligibility for an American visa," he said. "Violence has no place in democratic elections, and I can guarantee you that the perpetrators of such violence would not be welcome in the United States of America."

The U.S. is ready to increase its assistance to Nigeria in the fight against Boko Haram, and for that reason he said Washington will be watching the election closely.

"We are prepared to do more, but our ability to do more will depend to some degree on the full measure of credibility, accountability, transparency and peacefulness of this election," he said.

Jonathan visited Maiduguri recently, but he has been accused of ignoring the advance of Boko Haram, which has wreaked havoc in parts of Nigeria.

Earlier this month, Boko Haram took responsibility for a massive attack on the town of Baga, where activists say up to 2,000 people were killed.

Amnesty International released satellite images showing what appear to be thousands of destroyed structures in the town following the attack.

Boko Haram, which means "western education is forbidden" in the local language, has waged a bloody battle to impose Islamic law throughout Nigeria.

Last April the group kidnapped around 270 school girls from the town of Chibok, sparking worldwide outrage.

The Associated Press contributed to this report.