Boko Haram forces appear poised to attack Maiduguri, a city of 2 million in northeast Nigeria –- meaning that 200,000 Christians could be at risk of slaughter by the Islamist terror group, say U.S. intelligence officials and experts on Nigeria.

"An attack on Madiguri is very likely," said J. Peter Pham, director of the Africa Project at the Atlantic Council, echoing U.S. intelligence officials. Pham believes, as do other experts, that Boko Haram has already placed "sleeper cells" among the tide of refugees who have fled the group’s murderous rampage through Africa's most populous nation. "They've done it everywhere else they've gone,” said Pham. “So why not Maiduguri?"

One big concern is the large number of Christians in the city -- about 200,000, most of them Roman Catholic. In previous attacks, Boko Haram has offered Nigerian Christians the opportunity to convert or be killed. Already, 200 Christian churches have been lost to the group's onslaught.

Strategically, success in Maiduguri would provide Boko Haram with a launching point for further attacks on the neighboring states of Cameroon, Chad and Niger, all of whose territory were once part of an Islamic caliphate that lasted six centuries, ending in the 1300's. Boko Haram declared its own caliphate in the region last year.

But even with limited success, the image of Boko Haram’s black jihadist flag flying over any part of Maiduguri could be a symbolic victory. "Even if they only raise the black flag only briefly, it will be a big blow to the central government," said Pham.

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Boko Haram currently has about 4,000 to 6,000 men --and kidnapped boys -- under arms, and its tactics have improved. "They're operating at a faster tempo and on a larger scale. They are now capable of large-scale operations," said a U.S. official, citing last month's deadly assault on 16 villages and the city of Baga, northeast of Maiduguri. By some estimates, 2,000 people died in those attacks.

There is evidence that the group has already begun testing defenses on Maiduguri’s outskirts. A senator from Borno State, Ahmed Zanna, told NBC News Thursday that a group of suspected Boko Haram militants raided the village of Mbuta, just 15 miles northeast of Maiduguri, killing eight people, burning buildings and forcing most others to flee their homes. “Some people were burned alive,” said Zanna, who is currently inside Maiduguri. He also said that on Thursday more than a dozen people were killed in a suicide blast in Biu, 100 miles to the southwest.

Zanna told NBC News that despite these attacks residents were determined not to let their city be overrun. “If Boko Haram comes back people will be ready,” he said. “They will come out in the streets in their hundreds and thousands to defend Maiduguri.”

John Campbell, the former U.S. ambassador to Nigeria and now director of the Bunche Center at the Council on Foreign Relations, said an internal CFR analysis shows that there is only one road in and out of Maiduguri that isn't controlled by the group.

"It's a noose around Maiduguri," said Campbell, citing recent attacks on villages along Lake Chad that cut off access to the north and east. "There's one road open, going west. There is potential for a raid and occupation."

Most of those who spoke to NBC News think an attack could be timed to the country's rescheduled elections. Last Saturday, the National Election Commission postponed scheduled national elections from February 14 until March 28, but claimed that the inauguration of a new president will still be held May 29.

If and when Boko Haram makes a move on Maiduguri, said a senior U.S. intelligence official, the terror group will have one great tactical advantage. It is very familiar with the city. Late last decade Boko Haram set up an encampment near the so-called Railroad Mosque, which is next to the train station on the western edge of the city. Federal troops launched an attack on the group’s camp in 2009, killing its then-leader Mohammed Yusuf. Yusuf, now seen as a moderate, was replaced by current leader Abubakar Shekau, who has run the recent campaign of terror.

Campbell believes the initial Boko Haram raids will focus on the area around the Railroad Mosque in order to enhance the group's narrative. Intelligence officials and Nigeria experts think it would be difficult for Boko Haram to hold the city, but Boko Haram has surprised both officials and experts in the past.