Russia says it will take extraordinary measures to combat any terrorist threat when it hosts the World Cup next year, as its military campaign in Syria makes the country a prime target for jihadists.

A bombing on the metro in Saint Petersburg in April that left 15 people dead was among the recent high-profile terror attacks on Russian soil.

The fear of more attacks was heightened after seven people were stabbed in Siberia in August in an attack claimed by the Islamic State (IS) group, while the authorities have reported breaking up several jihadist cells across the country.

"There is a very real threat of an attack in Russia" during the World Cup, which runs from June 14 and July 15 2018, said Alexander Golts, an independent Russia expert specialising in security.

Russia has experienced a number terror attacks over the last 20 years and during two wars in Chechnya, but the threat has increased since Moscow's military intervention in Syria in September 2015 to support President Bashar al-Assad's regime, making it a priority IS target.

"The authorities say they have succeeded in destroying IS. But several thousand Russians have been taking part in (jihadist) conflicts and now they are beginning to return to Russia," Golts told AFP.

According to the Federal Security Service (FSB), about 2,900 Russian jihadists, most of them from the Muslim-majority Caucasus republics, have fought in Syria. Between 2,000 and 4,000 more fighters from Central Asia now live in Russia.

The world's most high profile sporting event, along with the Olympic Games, makes an ideal terrorist target.

- Threats against players -

Every day, dozens of calls to commit attacks during the tournament from IS propaganda organs are published via social networks. Many of these involve threats against players.

But Pascal Boniface, director of the Institute of International and Strategic Relations in Paris, said these threats are put out "to attract attention".

There is a risk of terror at "any global sporting event which attracts cameras and those with a desire to make an impact".

Several months before the Winter Olympics in the Black Sea resort of Sochi in 2014, bomb attacks killed 34 people in Volgograd, also in southern Russia, and "we were very worried about attacks" during the event, Boniface said.

FSB director Alexander Bortnikov revealed last month that a number of planned attacks had been thwarted ahead of the World Cup warm-up event the Confederations Cup in June.

- Human rights concerns -

In 2016 Russia set up an international "working group" charged with overseeing safety at the World Cup and bringing together the security services of 32 countries, despite diplomatic tensions between Moscow and the West, Bortnikov said.

"Every country is sending delegations and the common cause of the fight against terrorists goes beyond any disagreement they might have," Boniface added.

Saint Petersburg police last week began deploying dogs trained to sniff out explosives on the underground network, while a list of sensitive areas which will be kept under particularly close watch -- including luxury hotels, training grounds and tourist areas of host cities -- was published.

Although the authorities remain tight lipped on the details of the World Cup security plan, deputy prime minister Vitaly Mutko has announced that at least 30 billion rubles (445 million euros, $512 million) will be spent by Russia in this area.

A decree concerning "strengthened security measures during the Confederations Cup and the World Cup" signed by President Vladimir Putin came into force this June and will be applied again from May 25 to July 25 next year.

The decree includes measures limiting the right to protest and curbing driving in the tournament's 11 host cities, as well as introducing no-fly zones and forbidding entering these cities by bus without a special permit.

Human Rights Watch said it was "impossible not to be concerned" by aspects of the increased safety measures.

"We have already seen the consequences of the presidential decree," its Russia researcher Yulia Gorbunova told AFP, pointing to the 33 people arbitrarily detained during the Confederations Cup.

"There are reasons to believe that during the World Cup (these restrictions) will be all the more severe," she said.