PARIS (Reuters) - French police on Wednesday arrested a teenager suspected of plotting to kill in the name of the Islamic State, sources said, bringing the number of 15-years-olds detained on such grounds to three in the last week.

The latest youth detained is believed to have been planning an imminent attack and using encrypted social media channels to communicate with a French Islamist militant believed to be in Syria or Iraq.

In an operation led by France's domestic intelligence agency, police arrested the teenager in eastern Paris.

"We're working with extreme intensity to identify those we think are likely to carry out an attack," Interior Minister Bernard Cazeneuve told reporters, adding that Islamic State was recruiting "younger and younger individuals".

One of the three youths was said to have been planning an imminent attack when he was arrested on Saturday. A source said on Wednesday that another 15-year-old had been arrested last week.

A source inside the prosecutor's office said the youngsters had used the Telegram encrypted messaging system to communicate with Rachid Kassim, an Islamic State jihadist of French nationality.

Their arrests follow the detention of three women, including a 19-year-old, who had allegedly wanted to attack a Paris railway station using a car laden with gas cylinders.

France is reeling from a wave of militant attacks that have killed more than 230 people since January 2015, and its intelligence services are struggling to dismantle a web of militant networks inside the country.

In November last year, a squad of suicide bombers and gunmen killed 130 people in a coordinated attack on multiple sites in Paris.

In July, a Tunisian delivery man killed 86 people when he drove his truck through a crowd on Nice's seafront. Later that month two militants knifed to death an elderly priest at his altar in a church in northern France.

"What sets France apart (from other European nations) is the wide-open profile of recruit: urban or rural, Muslim or convert, man or woman," said Arnaud Danjean, a European Parliament lawmaker who specializes in defense and security.

"Acts like the attempted attack with gas cylinders, the throat-slitting of a priest, or the guy who ploughs his truck through a crowd, that today is the face of this threat."

In a separate case, police were on Wednesday questioning three Syrians and two Kuwaitis suspected of trying to buy assault rifles, a source in the prosecutor's office said. There was no sign of a link to terrorism at this stage, the source said.

(Reporting by Emmanuel Jarry; editing by Andrew Roche)