A string of murders involving the same weapon have raised concerns that a serial killer may be on the loose in a Paris suburb.

The killings come as France is still reeling from a wave of terror-related shootings in its south.

Four murders have taken place in the Essonne region of the capital since November, the latest that of a 47-year-old woman on Thursday.

The same semi-automatic gun was used in all four attacks, prosecutor Marie-Suzanne Le Queau said during a press conference.

Authorities stressed that there is no evidence yet of a link to terrorism in the killings, but interior minister Claude Gueant told French radio Europe-1 that he fears a serial killer might be behind them, and that "maximum resources" would be used to find the perpetrator.

French agencies already are on alert after a string of shootings last month in southern France that police say was the work of an Islamist militant who used a motorbike, video-taped the shootings and claimed links to al Qaida. The alleged gunman was shot dead in a stand-off with security forces.

Ms Le Queau added that similarities were especially apparent in the last three murders, in which the victims were shot in the head. Gun cartridges were recovered in the most recent killing, when the gunman fled on a motorbike.

In the killings, Ms Le Queau said all four murders appeared to have been committed at the same time of the day, between 4pm and 6pm.

The first murder, which took place in November, involved a different method, but the same gun, Ms Le Queau said. She said a suspect in the first attack was already behind bars, but had retracted his confession to the November murder.