When Nigeria's new chief of defence staff was appointed earlier this year, he promised a swift end to the deadly violence being waged by Boko Haram Islamists.

"If we do our work cohesively, I can tell you we will finish that thing (the counter-insurgency) in no time," Air Marshal Alex Badeh said at his investiture on January 20.

Eight months on, Boko Haram look stronger than ever, having seized towns and villages in Nigeria's northeast at a rate which has drawn comparisons to Islamic State militants' rapid gains in Iraq.

The military in contrast seems far from cohesive: some soldiers have reportedly refused to deploy, complaining they are ill-equipped to fight the better-armed rebels.

Hundreds of others are said to have shouldered arms and fled their posts; salaries have reportedly gone unpaid; and soldiers left without proper food. The military denies the claims.

On Thursday, US Assistant Secretary of State for African Affairs Linda Thomas-Greenfield warned that the worsening security situation must be reversed.

"The reputation of Nigeria's military is at stake," she told a bilateral security meeting in Abuja. "But more importantly, Nigeria's and its children's future is in jeopardy.

"Failure is not an option."

- 'It is shameful' -

Africa's most populous nation and leading economy also has one of its largest militaries.

There are 80,000 personnel on active service and 82,000 paramilitaries, according to the International Institute of Security Studies' "The Military Balance 2014".

Out of this year's federal budget of 4.962 trillion naira ($30 billion, 23 billion euros), 968 billion naira or nearly 20 percent went to defence -- the highest since the 1967-1970 civil war.

Boko Haram is estimated to have between 6,000-8,000 fighters and is largely reliant on criminality for funding and looting the places it attacks, including military barracks.

Why the militants appear to have the upper hand has left many Nigerians baffled and politicians demanding answers.

"This is not the military that we used to know," said one former officer, who participated in Nigeria's first military coup in January 1966.

"How can a rag-tag group of dissidents overpower trained Nigerian soldiers? It is shameful," he told AFP. "Our military are just wallowing in self-denial."

- Years in the making? -

Nigeria's military woes are all too predictable for some.

Former army general-turned-lawmaker Ahmed Saleh believes the rot set in after a failed coup attempt against military ruler Ibrahim Babangida in 1990.

Babangida got rid of experienced senior officers and handed more control to the military high commands in Abuja, leading to "decay" in the ranks in terms of training and skills, he said.

Front-line operations were impossible without weapons and ammunition shortages, he was quoted as saying in Nigeria's media on Friday.

"We have a duty to rebuild the armed forces and unless we understand these basic facts, we are not going anywhere," he added.

The IISS said that despite refit and repair programmes in recent years, much of Nigeria's defence equipment is "unfit to be deployed for prolonged periods of time".

Procurement has not focused on counter-insurgency while analysts blame widespread graft and management failures for the lack of improvement, despite the increases in defence spending.

Some reports have said less than $100 million of the nearly $2 billion defence budget actually gets to deployed troops.

Ryan Cummings, chief Africa analyst with Red24 risk consultants, said the fact that members of Nigeria's "anti-Boko Haram" unit -- the 7th infantry division -- had refused to deploy was telling.

It showed "just how poorly the insurgency is being addressed from a military perspective", he said.

- Demoralising effect -

Virginia Comolli, a West Africa security and extremism specialist at the IISS, said she was unsurprised that the military was demoralised and struggling to defeat Boko Haram.

The better equipped rebels were gaining confidence and beginning to fight more like a conventional army, "intimidating (and possibly) overwhelming... the soldiers battling them", she added.

Rights abuses, including extra-judicial killings, arbitrary detention and torture, have meanwhile eroded civilian trust in the military, losing them a vital intelligence asset.

On Friday, the military indicated that it recognised the seriousness of the situation, calling the insurgency a threat to national sovereignty that needed to be reversed.

Cummings suggested, however, that regional support was now more important than ever, given the frequent cross-border raids from camps outside the country.

Without regional help, "it is likely that Boko Haram will continue to expand its operational presence across Nigeria's northeast and possibly into neighbouring countries", he said.