By Alexandra Hudson and Sabine Siebold

BERLIN (Reuters) - Radical Islam poses a critical security threat to Germany, Interior Minister Thomas de Maiziere warned on Tuesday, saying the number of people capable of staging attacks in the country stood at an all-time high.

Besides the risk posed by German jihadists returning from Syria, there was also the danger of violent clashes on German streets as rival extremist groups turn on each other - mirroring the conflicts of the Middle East, he told a security conference.

De Maiziere said security forces believed the greatest danger came from radicals striking out alone, as happened in Canada last week, when two soldiers were killed in attacks that police said were carried out by recent converts to Islam.

"The situation is critical. The number of threatening individuals has never been as high as now," he said. "We represent freedom, and are therefore an object of hate."

The domestic intelligence agency (BfV) has warned that ultra-conservative Salafism was becoming increasing popular -- boosting the number of potential recruits for Islamic State.

Some 450 people have traveled from Germany to join the jihadists in Syria and Iraq. Around 150 have returned. The German authorities are monitoring a total 225 suspects believed capable of launching attacks on domestic soil, compared to just 80 or 90 a few years ago, de Maiziere said.

Although not directly involved in the U.S.-led air strikes on Islamic State in Syria and Iraq, Germany has agreed to send weapons to Kurdish forces in northern Iraq to help them defend themselves against the radical militants.

Meanwhile, IS has released propaganda videos in German, with some featuring native-German speaking jihadists who threaten to unleash attacks back home.

In various German cities in recent months, gangs of Salafists and local Kurds have fought street battles, tensions fueled by Islamic State attacks on Kurds in the Middle East.

On Sunday, a group of 4,000 far-right supporters staged an anti-Salafist march in the western city of Cologne, pelting police with stones, bottles and fireworks, injuring 49 officers.

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"We are concerned that violent clashes between extremists on our streets could escalate," BfV president Hans-Georg Maassen said in a statement.

The BfV calls Salafism, which espouses a strict, puritanical form of Islam, Germany's "most dynamic Islamic movement" and estimates the number of Salafists rose from around 3,800 in 2011 to 5,500 last year and could reach 7,000 by the end of 2014.

Separately, Germany's BND foreign intelligence agency said on Tuesday that Islamic State will be able to mount operations in Iraq "for the foreseeable future" despite the air strikes and efforts by Iraqi security forces to regain territory.

In Syria, the BND said fighting between IS and Kurdish forces in Kobani near the Turkish border showed that the militants were still in a position to attack, even if their mobility had been impaired by repeated air attacks.

(Additional reporting by Sabine Siebold; Editing by Gareth Jones and Crispian Balmer)