Police in northeastern Kenya say an attack has killed at least five people including three police officers near the Somali border.

Authorities said gunmen opened fire and threw an explosive device late Wednesday in Wajir. They said at least 13 people were missing after the attack.

No group claimed responsibility, but Kenyan officials have blamed similar attacks on Somalia's Islamist al-Shabab rebels.

Kenyan forces entered Somalia in October to push back al-Shabab, a militant group Kenya blames for a series of cross-border kidnappings.

Meanwhile, a human rights group says Kenyan security forces have carried out beatings and arbitrary detentions near the Somali border.

New York-based Human Rights Watch released a report Thursday, a day after it says security forces rounded up and beat civilians in the eastern town of Garissa.

The report describes a series of abuses mostly targeting ethnic Somalis.

Human Rights Watch says the worst abuses took place at the Dadaab refugee camp, which is home to more than 400,000 Somali refugees. It says police responded to attacks against security forces there in December by beating refugees, carrying out arbitrary arrests and raping women.

The report says the Kenyan military pledged to investigate the group's allegations.