After three days of bloodshed that has left 17 people dead, French security forces are desperately searching for the former partner of one of the three Islamist gunmen killed by police.

Hayat Boumeddiene, described by police as armed and dangerous, is the ex-girlfriend of Amedy Coulibaly, who died on Friday evening when heavily armed elite forces stormed a Jewish supermarket in northern Paris where he was holding at least 15 people hostage.

Coulibaly had killed four shoppers when he entered the kosher store on the Avenue de la Porte de Vincennes carrying two Kalashnikov assault rifles, the Paris prosecutor's office said. Agence France-Presse reported he had telephoned friends during the ensuing siege, demanding that they mount further attacks.

President François Hollande called an emergency cabinet meeting this morning, having warned the night before that French people should be prepared for more violence and urged "vigilance, unity and mobilisation".

In a second, near-simultaneous assault with stun grenades and assault rifles, French special forces also shot dead Chérif and Saïd Kouachi, the brothers behind the killing on Wednesday of 12 people, including two police officers, at the satirical magazine Charlie Hebdo in Paris.

After a brief shootout at a road block, the brothers had been cornered since early Friday morning in the warehouse of a small printing business on an industrial estate in Dammartin-en-Goële, about 25 miles north-east of Paris.

Reportedly armed with Molotov cocktails and a rocket-propelled grenade launcher, they took the manager of the company hostage but later let him go after he treated Saïd for a neck injury. A second worker stayed undetected on the second floor of the warehouse throughout the siege, sending text messages to the forces surrounding the building.

Similarly, up to five people, including a three-year-old boy, survived inside a refrigerator for five hours at the Porte de Vincennes kosher supermarket, the prosecutor's office said. Police were reportedly able to pinpoint their location by their mobile phones.

As the hunt intensified for Boumeddiene, wanted in connection with Coulibaly's fatal shooting of a female police officer in Montrouge on Thursday, a badly shaken France prepared for a march of national unity in Paris on Sunday.

The British prime minister, David Cameron, and his German, Spanish and Italian counterparts, Angela Merkel, Mariano Rajoy and Matteo Renzi, as well as Maltese Prime Minister Joseph Muscat were due to attend.

But questions were starting to be asked about how the three men, all with known jihadi links, had managed to evade intelligence agencies. The trio had been in telephone contact more than 500 times over the three days, police said.

The French prime minister, Manuel Valls, said the twin attacks indicated there had been "clear failings" in intelligence.

Chérif Kouachi, 32, was jailed for 18 months for his role in a network, of which Coulibaly was also a member, that sent volunteers to fight alongside al-Qaida in Iraq between 2003 and 2005. His brother Saïd, 34, travelled in 2011 to Yemen, where he received weapons training from al-Qaida Yemen.

Coulibaly, 32, reportedly met Chérif in prison. Both men, described by L'Obs news weekly as "very close", were implicated in a 2013 plot to break an Algerian Islamist, Smaïn Aït Ali Belkacem, out of the Paris high-security prison where he was serving a life sentence.

BFM TV revealed on Friday night that it had been in telephone contact with Chérif from inside the printing warehouse earlier in the day. In a calm, assured voice, the gunman could be heard saying he had been sent by al-Qaida Yemen.

Saying it had not released the tapes earlier to avoid compromising the police operations, BFM TV also replayed a conversation it had with Coulibaly in which he said the attacks against Charlie Hebdo and the Montrouge police officer had been synchronised. He also said he had targeted the Paris supermarket "because it was Jewish", and claimed to be a member of Islamic State.

Overnight, al-Qaida in the Arabian Peninsula's top sharia official, Harith al-Nadhari, threatened France with further attacks, the SITE monitoring group reported. "It is better for you to stop your aggression against the Muslims, so perhaps you will live safely," he said. "If you refuse but to wage war, then wait for the glad tiding."