UPDATE 4-Two terrorist suspects arrested on plane in Germany

(Adds security sources, analyst)

By Sabine Siebold

BERLIN, Sept 26 (Reuters) - German police boarded a Dutch airliner at Cologne airport on Friday and arrested two men suspected of planning to take part in terrorist attacks.

A police spokesman identified the suspected Islamist militants, on a KLM aircraft about to take off for Amsterdam, as a 23-year-old Somali and a 24-year-old German born in Somalia's capital Mogadishu.

"It all went off in quite an unspectacular manner," he told Reuters television.

Police in the North Rhine-Westphalia region of western Germany said they suspected the men were planning a violent form of jihad, or holy war, and had found farewell letters.

"It looks as if they were on their way to a terrorist training camp, possibly in Pakistan or Afghanistan," said terrorism expert Rolf Tophoven.

Somalia is riven by a civil war pitching Islamist rebels against an Ethiopian-backed government. A large number of Somali refugees have moved to western Europe over 17 years of conflict.

"The origin of the (suspects) is not surprising because Somalia has for some time been a place where al Qaeda commandos stay and make preparations," said Tophoven.