As Kenyan troops make slow progress against Islamist rebels in war-torn southern Somalia, experts and officials are warning of the risk of revenge attacks from home-grown militants back home.

Almost two weeks since its forces crossed into Somalia to battle the Al Qaeda-linked Shebab, Kenya has already suffered grenade attacks blamed on the extremist militia.

Two grenade blasts earlier this week in Nairobi killed one person and wounded dozens of others.

Another four people meanwhile were killed by a rocket propelled grenade near the Somali border, an attack also blamed on Shehab fighters.

A 28-year old man from western Kenya was sentenced to life imprisonment Friday for his role in the grenade attacks and being a member of the Shebab The police said he had been trained in Somalia earlier this year.

"We knew that there would be repercussions within Kenya itself," said police chief Mathew Iteere.

"We have credible information there is quite a number of young people who have crossed into Somalia to fight for Al Shebab," he added.

"These are Kenyans, not necessarily of Somali origin... We are talking about the bad elements from any of the communities."

On Thursday, top Shebab commander Mukhtar Robow Ali called on Kenyan operatives to launch attacks.

"Now is the time to act... there is a need to make a huge explosion," he said.

Andrews Atta-Asamoah, a Nairobi-based member of South Africa's Institute for Security Studies, warned that radicalised youth were inspired by Shebab orders rather operating under their command.

"The threat is becoming more amorphous because he (Robow) asked them to take individual decisions... We should be expecting individual actions from sleeping cells," he said.

"They (the Shebab) are not necessarily giving direct instructions to the group, but they are counting on the group to rise to the challenge on the ground."

International experts have long warned of the risk inside East Africa, linking Kenyan and Ugandan operatives for July 2011 bombings in the Ugandan capital Kampala that killed at least 76 people, claimed by the Shebab.

J. Peter Pham of the Washington-based Atlantic Council, gave a similar warning.

Even if Kenya was able to smash rebel bases in Somalia, Nairobi should also be worried about threats "already present deep inside the country," he said.

Attacking Shebab bases in Somalia raised the "risk of ‘sleeper cells’ or even self-radicalised ‘lone wolf’ sympathisers to respond by carrying out attacks within Kenya," Pham said.

The UN Monitoring Group on Somalia and Eritrea warned in a July report that Kenyans constituted the "largest and most structurally organised non-Somali group within Al-Shebab."

In the past, Kenyan supporters were based primarily within the ethnic Somali community, the report added.

But since 2009 the Shebab had "rapidly expanded its influence and membership to non-Somali Kenyan nationals."

It detailed evidence of an "extensive and complex financial support system" it said showed the recruitment of fighters and fundraising.

Some of that was done through a mosque that owned the land of Kenya's largest second-hand clothes market, it alleged.

One Kenyan, Ahmad Iman Ali, who heads the Nairobi-based Muslim Youth Centre, was "believed to command an estimated force of between 200 and 500 fighters, most of whom are Kenyans," the report said.

The Shebab not only pose an "increasingly acute regional and international threat" but are also "giving rise to a new generation of East African jihadist groups," it added.

Kenya's unprecedented military incursion into Somalia was to fight rebels it blames for attacking its territory and abducting foreigners inside the country.

But the operation has raised fears about the risk from Shebab sympathisers within Kenya's some two million ethnic Somalis and half-a-million Somali refugees.

"If the head of the snake is right here in Eastleigh, why did we go to Somalia?" the Daily Nation newspaper asked earlier this week, referring to a Nairobi district dominated by ethnic Kenyan Somalis.

"Somebody should explain why we are pursuing Al-Shebab militants deep inside Somalia instead of taking out the nexus right here at home."