Somali rebel group al-Shabab denied responsibility for the deadly bomb attacks in Nairobi, Kenya on Saturday.

At least six people were killed and 63 injured in the Kenyan capital after passengers in a moving car threw grenades at a busy bus station. Kenya officials blamed the Islamic militant group, which is thought to have been behind a string of attacks in Nairobi that started in October, when Kenya troops invaded Somalia.

“We are not actually involved in such low-scale attacks. Al-Shabab has nothing to do with the hand grenade attacks that hit Nairobi,” senior militant leader Sheik Mohamed Ibrahim said, according to the Agence France Presse news agency.

But the denial didn't stop the rebel group from using the bombing to taunt the Kenyans.

The turmoil that is rapidly spreading across Kenya is in fact a monument to the grossly misguided foreign policies and misplaced priorities of the Kenyan government that seems to be under the delusion that it is waging a winnable war against Islam, al-Shabab said in a follow up statement.

It's oddly amusing! Kenyan blood splatters at home and abroad whilst nurses still whine about bread and butter -- but hey, does it all matter! the group said via its Tweeter feed, referring to an ongoing health workers strike in Kenya.

Kenyan police, who are still certain that al-Shabab was responsible, have arrested four suspects, including a man identified as Sylvester Opiyo and three teenagers.

The attack came after Kenyan officials said the army would join with the 12,000 African Union (AU) forces in Somalia, for which al-Shabab promised to retaliate.

There will be a day we will hit their cities if they continue with their aggression, Ibrahim told AFP.

The AU peacekeepers expelled al-Shabab from Mogadishu last year and have successfully kept the Islamic rebels out of the capital city since.

Over the past year, Al-Shabab has admitted to a number of attacks, as well as gun battles with the Kenyan and Ethiopian military. However, the group never took responsibility for bombings in Nairobi similar to what happened on Saturday.

This is an attack by people who think they can puncture the resolve of Kenyan people to fight against terror, Kenyan Vice-President Kalonzo Musyoka told reporters from the Kenyatta hospital.

It is a difficult moment, but it should strengthen our resolve.