Local media reports and eyewitnesses say gunmen have attacked a vehicle in northeastern Kenya, killing at least four people.

The reports say the attack happened in the sparsely populated northeastern district of Mandera, near the Somali-Kenya border.

It is not immediately clear who is responsible for the attack, or who was being targeted.

The incident is the latest in a series of attacks throughout the country, following Kenya's military offensive into Somalia to pursue al-Shabab insurgents, who Kenya blames for several cross-border kidnappings of foreigners.

On Wednesday, a Kenyan man – who admitted to being a member of al-Shabab – pleaded guilty to one of two grenade attacks that took place in Nairobi earlier this week. The explosions killed one person and injured more than 20 others.

Al-Shabab has warned it will carry out attacks inside Kenya in response to the Kenyan army offensive.

Meanwhile, the prime minister of Somalia's transitional government says he supports Kenya's military operation, despite earlier comments by Somalia's president casting doubt on the government's support for the move.

On Monday, President Sheikh Sharif Sheikh Ahmed said his government opposes Kenyan military action in Somalia, and that only African Union troops can operate legally in the country.

But Somali Prime Minister Abdiweli Mohamed Ali said late Wednesday that Somalia supports “any self-defense action” Kenya takes against al-Shabab, saying that his country is “very grateful to Kenya.”

However, Prime Minister Ali said Somalia is taking the lead in all operations carried out inside his country.