The suspected Islamic State militants holed up in a northern Paris apartment following a raid by French police had planned an attack on La Defense business district in Paris, a source close to the investigation said on Wednesday.

Gunfire and explosions shook the Paris suburb of Saint-Denis early on Wednesday as French police surrounded an apartment where a Belgian Islamist militant suspected of masterminding last week's attacks in the French capital was believed to be holed up.

Two assailants were killed, including a woman who detonated a suicide bomb, a source close to the case said, adding that the police operation was continuing to flush out two other suspects.

The target of the raid, which filled the streets of St Denis with heavily armed police and soldiers, was Islamic State militant Abdelhamid Abaaoud, who was initially thought to have orchestrated the Paris attacks from Syria, police and justice sources said.