The United States has warned its citizens in Kenya against terrorist attacks ahead of visit by Obama and as suspected militants from the Shebab extremist group killed at least five people in a roadside bomb attack.

The United States has warned its citizens in Kenya against terrorist attacks ahead of visit by President Barack Obama and as suspected militants from the Somali-led Shebab extremist group killed at least five people in a roadside bomb attack in the country.

The US president is scheduled to give a speech to the Global Entrepreneurship Summit (GES) in Nairobi, which opens on July 24.

"As with all large public events, there is the opportunity for criminal elements to target participants and other visitors," the US State Department said in a statement.

"Large-scale public events such as this summit can also be a target for terrorists. US citizens should maintain a high level of security awareness."

The US embassy in Nairobi, in its previous warning last issued in May, said its citizens should be "aware of continuing and recently heightened threats from terrorism and the high rate of violent crime in some areas."

Obama's visit later this month will be his fourth to Africa since becoming US president, but his first to Kenya since taking office in 2009, his father's birthplace.

Meanwhile, police said on Tuesday that the attack on a police truck took place on Monday in the eastern coastal county of Lamu, which borders Somalia and has been hit by a wave of attacks by the Al-Qaeda linked group.

"Five people were killed in the attack, including a mother and her child," Lamu police chief Chrispus Mutali said, adding that five others were wounded in the attack.

All those killed were reportedly civilians who had been given a lift by the police, while the officers escaped unharmed.