Still image taken from video shows Police vehicles at the scene near where a French police commander was stabbed to death in front of his home in the Paris suburb of Magnanville, France, June 14, 2016. [Photo/Agencies]

PARIS/LES MUREAUX - A suspected Islamist attacker stabbed a French police commander to death outside hishome and later killed his companion, a policewoman, in an attack claimed by Islamic State and denounced by the government as "an abject act of terrorism".

The assailant, a 25-year-old Frenchman of Moroccan origin,was jailed in 2013 for helping Islamist militants go to Pakistan and had been under security service surveillance, including wiretaps, at the time of the attack, police sources said.

The attacker filmed part of the assault live on the social network Facebook, according to David Thomson, a journalist specialised in radical Islamists. In his Facebook message, he linked the attack to the Euro 2016 soccer tournament now underway in France, saying: "The Euros will be a graveyard."

The attacker, named by police and justice sources as Larossi Abballa, knifed the 42-year-old commander repeatedly in thestomach on Monday evening.

He then barricaded himself inside the house in Magnanville, a suburb some 60 km (40 miles) west of Paris, taking the man's 36-year-old partner and their three-year-old son hostage.

Police commandos shot Abballa dead when they stormed the house after negotiations failed but they found the woman, a secretary at a police station in a nearby suburb, killed with a knife, a source close to the investigation said.

The boy was unharmed but in a state of shock.

"An abject act of terrorism was carried out yesterday in Magnanville," Interior Minister Bernard Cazeneuve said after an emergency government meeting, before visiting Les Mureaux, wherethe police commander worked.

President Francois Hollande said the killings were "undeniably a terrorist act" and that the terrorist threat in France was very high.

Police searched Abballa's home and other locations on Tuesday and detained two people close to him for questioning, apolice source said.

The killings came as France, which has been under a state of emergency since Islamic State gunmen and bombers killed 130 people in Paris last November, was on high security alert forthe Euro 2016, which began last Friday.

Police are under "extreme pressure" and "close to burn-out," the head of FO labour union Jean-Claude Mailly told France 2 television.