ABUJA (AFP) - A bombing blamed on Boko Haram in the heart of Nigeria's capital raised fears on Thursday of a worsening Islamist insurgency, with security forces struggling to prevent attacks in remote villages and near the seat of government.

Wednesday's blast, which killed at least 21 people, shook the crowded Emab Plaza in downtown Abuja during the afternoon rush as shoppers were buying groceries an hour ahead of the country's World Cup match against Argentina.

The explosions struck "a very prominent street and it sends a very loud message", said Mr Nnamdi Obasi, a Nigeria researcher at the International Crisis Group. "The message is that everywhere in the city is vulnerable."

Boko Haram's five-year uprising to create an Islamic state in northern Nigeria has killed thousands but there were hopes earlier this year that the violence had been contained in the remote north-east, the group's stronghold.

An April 14 bombing at a bus station on the outskirts of the capital and copycat attack at the same spot on May 1 cast doubt on claims that the insurgents had been weakened.

"The security situation in the northeast is very grim and the return of bombings in Abuja really raises questions about how much progress there has been against the insurgency," said Mr Obasi. "It is embarrassing for the government."

Dozens of soldiers and police guarded the Emab Plaza on Thursday, with the main road in front closed off, traders denied access to their shops and the burnt out shells of cars littering the blast zone.

Shellshocked shopkeepers and witnesses swapped stories of near misses as they returned to the scene.

"I ran after a customer who was at that gate to give him his phone which he forgot in our shop," said trader Suleiman Mohammed.

"I saw a large crowd of people there. The bomb exploded before I got back to the shop."

The police and the country's National Information Centre said on Wednesday one suspect had been arrested after the explosion, while another was shot dead by troops as he tried to escape on a motorbike.

The area, sandwiched between two other shopping centres and one of the busiest in central Abuja, was littered with body parts in the immediate aftermath and soaked in pools of congealed blood.

Shopkeeper Osaretin Odafe spoke of seeing "many bodies dismembered".

A security officer for a foreign company who asked to remain anonymous said his firm was advising employees in Abuja to stay home aside from going to work and "to avoid crowded places".

Analysts say the advice underscores the broader security decline in Nigeria's capital, which had been spared Boko Haram violence for nearly two years prior to the April 14 attack.

The Boko Haram conflict has received unprecedented global attention in recent weeks following the kidnapping of more than 200 schoolgirls in April, which sparked worldwide outrage.

Mr Ryan Cummings, a South Africa-based security analyst for Red 24, said the latest attack, if confirmed to have been carried out by Boko Haram, would suggest that the group had the ability to bypass security "even in the most well-guarded of areas".

"Discrediting and undermining the Nigerian government in terms of both domestic and international onlookers may very well be the motivation for these attacks," he told AFP.

Mr Cummings said the attack would spread fear outside of Abuja, "extending to cities such as Lagos which have been spared from the violence this far".

The warning has already been borne out by a minor explosion at a fuel depot in the port at Lagos which quickly led to hysterical reports of a terrorist attack across social media despite being caused by a gas cylinder catching fire.

Nigeria has been waging an offensive in the north-east since May last year to crush the uprising, but the operation has been widely criticised as failing to stem the unrest, with more than 2,000 people killed already this year.

Mr Andrew Noakes, who coordinates the Nigeria Security Network of security analysts said Boko Haram was intent on showing Nigeria that the insurgency is not just "a northern problem".

"They want to send a message that nowhere is safe. But its significance should not be overstated.

"Abuja is not too far from some of the northern areas where Boko Haram has operated in force in recent years, and carrying out bomb attacks does not necessarily require a high level of sophistication or infiltration into the local population."