One police officer was killed when a grenade was hurled at a police van on Friday [Reuters]

Kenyan police have announced the arrest of 346 foreigners in the capital, Nairobi, after two separate grenade and gun attacks last week killed three policemen.

Anthony Kibuchi, Nairobi's provincial police chief, told Reuters news agency that 52 of those arrested were Ethiopians while the rest were of Somali origin.

"The security operation on aliens was carried out all over Nairobi," Kibuchi said on Monday.

"We have come out with more stringent measures to ensure holiday festivities are free of any incidents
and appeal to members of the public to co-operate with police."

In the first attack on Friday, unidentified men lobbed a grenade into a police vehicle in the capital's predominantly Somali suburb of Eastleigh, killing one officer. Later in the day, two men on motorbikes shot dead two traffic policemen when they had been pulled over.

FBI assistance

Kenya asked the US Federal Bureau of Investigation for assistance to investigate the incidents.

Some security analysts said sympathisers of neighbouring Somalia's al-Shabab group may have been behind the attacks.

But Mathew Iteere, the Kenyan police chief, ruled out the group's involvement.

"We do not think it's the al-Shabab militia group who are responsible," he said on Saturday.

And when contacted by the AFP news agency, Muktar Robow Abu Mansur, an al-Shabab leader in Mogadishu, denied his group had anything to do with the killings.

"We have no reason to target and attack people inside Kenya," he said, blaming "elements aiming to destroy the image of the Somali society" in the country. 

Kenyan police have been cracking down on illegal immigrants within Eastleigh in recent weeks and have also been targeting suspected Ethiopian rebels sheltering in the country.