Gunmen armed with grenades, AK-47s and ammunition belts opened fire inside the affluent mall during a children’s cooking competition, with witnesses saying the terrorists numbered up to 10 men – some of whom looked as young as 18.

Reinforcements arrived this morning, according to the ministry, with armoured vehicles bringing more soldiers to join in the rescue operation. At approximately 11am (9am BST), fresh barrages of gunfire were heard from within the shopping centre, reportedly as security forces ran in a line and crouched along the front of the building.

Speaking to reporters today, Interior Minister Joseph Ole Lenku said the police and army were in complete control of the situation, and that they now had remote control of the shopping centre’s CCTV systems.

Mr Lenku said the official number of injured had increased to 175, but he added that he was thankful for the fact that they had been able to get more than 1,000 people out of the mall safely.

Today’s movements at the scene come as security forces try to establish the number of hostages within the building – thought to be around 30 – with several of those trapped in the building overnight having been able to escape on their own.

Nairobi’s Police Chief, Benson Kibue, yesterday described the assault as a terrorist attack, and last night the Somalia-based group al-Shabaab was reported to have claimed responsibility.

According to eye-witnesses, the gunmen told Muslims to leave the centre shortly before midday and said that non-Muslims would be targeted.

Some claimed the attackers had asked, “who is Prophet’s mother?” in an attempt to discover non-Muslims. One man escaped by showing the attackers his ID with his name, Hakim, on it. Another reportedly failed to name the Prophet’s mother and was killed.

Grenade explosions and gunfire sent scores fleeing in panic from shops and restaurants on to the streets, according to witnesses and the Red Cross. Shooting continued hours after the initial assault as Kenyan troops surrounded the mall and police and soldiers combed the building, hunting down the attackers shop by shop. A police officer inside the building said the gunmen were barricaded inside the Nakumatt supermarket, one of Kenya’s biggest chains.

Kenya’s Deputy President, William Ruto, promised last night that they would “end the siege and get to the bottom of the attack”. “We will bring to account the perpetrators and their accomplices,” he said. Kenya Police said on Twitter that they had detained one of the Westgate Mall suspects, though reports later suggested he had died in hospital.

The Inspector General of Police, David Kimaiyo, said officers had rescued “quite a number” of hostages. He added that hostages were being screened as officers were “not taking any chances”.

Police initially said it was an attempted robbery but later acknowledged that as many as 10 terrorists were involved. Al-Shabaab vowed in 2011 to carry out a large-scale attack on Kenya in retaliation for Somali and Kenyan military operations against its insurgents. Its threatened targets included the shopping centre, which is popular with both affluent Kenyans and Western expats.

Last night, the Foreign and Commonwealth Office said it was “urgently” attempting to discover if any Britons were among those killed. It has updated its travel advice to the thousands of Britons who visit the capital each year, warning “British nationals should avoid the area”. The Foreign Secretary, William Hague, said he was “appalled” by the attack, adding: “Our urgent priority is the welfare of UK nationals in Kenya.”

Yesterday’s attack, if confirmed as a terrorist-inspired action, will be the biggest such attack in Kenya since al-Qa’ida bombed the US embassy in Nairobi in 1998, killing more than 200 people. In 2002, the same militant cell attacked an Israeli-owned hotel and tried to shoot down an Israeli jet in a co-ordinated attack in Mombasa.

Who Are al-Shabaab?

Islamist extremism has been on the rise in East Africa for the past decade, with al-Qa’ida-linked groups such as al-Shabaab behind a growing number of terror attacks.

Since the Kenyan army began helping Somali forces tackle al-Shabaab insurgents in southern Somalia in October 2011, Kenya has fallen victim to a string of retaliatory gun and grenade attacks.

Last month, four Kenyan police officers were shot dead in Garissa, near the Somali border, when 40 armed men suspected of belonging to al-Shabaab attacked a police post. In July, the group released two Kenyan government officials it had seized in a 2012 cross-border attack, after holding them hostage in Somalia for more than a year.

Between July 2011 and July 2012 there were at least 17 attacks involving grenades or explosive devices in Kenya, killing at least 48 people and injuring around 200. Four of these attacks occurred in Nairobi, and four in Mombasa. Targeted locations have included police stations and police vehicles, nightclubs and bars, churches, a religious gathering, shops and a bus station.

Al-Shabaab was also behind the twin bomb attacks in Kampala, Uganda, that killed at least 74 people in 2010. These were again connected to military actions against them in Somalia. Sheik Ali Mohamud Rage, the group’s spokesperson, said soon after the bombings: “We are sending a message to Uganda and Burundi, if they do not take out their Amisom [African Union Mission in Somalia] troops from Somalia, blasts will continue and it will happen.”

Today, the group added: “The Kenyan government turned a deaf ear to our repeated warnings.”

Emily Dugan