Kenyan police have released four people arrested in connection with deadly grenade attacks blamed on the Somali militant group, al-Shabab.

Lawyers for the suspects say the four were questioned about their involvement in the bombings and released on Monday. They say the men were asked to report to police again on Tuesday.

Those arrested included three teenagers and an adult, Sylvester Opiyo, who was previously arrested in December on suspicion of having links to al-Shabab.

Al-Shabab has denied it was responsible for Saturday's attack.

The attack targeted the outdoor terminal of a major bus station in the capital, Nairobi, killing at least six people. About 60 others were injured.

This is the latest in a string of gun, bomb and grenade attacks that have killed dozens of people since Kenya sent troops into Somalia in October to fight al-Shabab.

The militant group recently formalized an alliance with al-Qaida, and East African governments have declared the militants to be a regional threat.

Al-Shabab has been trying to overthrow Somalia's government and turn the country into a strict Islamic state.

The group claimed responsibility for bombings in Uganda's capital that killed more than 70 people in July 2010.