Fifteen people have been killed in attacks on churches in the Kenyan town of Garissa near Somalia, say the Kenyan Red Cross and a medical official.
Regional deputy police chief Philip Ndolo said balaclava-clad "goons" attacked the town's Catholic church and the African Inland Church (AIC).
A combination of grenades and gunfire was used, police said.
Kenya's border region has been tense since it sent troops into Somalia to pursue al-Shabab Islamic militants.
Kenya said the operations, launched last October, were designed to bring an end to kidnappings on Kenyan soil and other violence which it blamed on al-Shabab.
But since then, al-Shabab has been blamed for a further string of grenade and bomb blasts across Kenya - though it has never admitted to carrying out any such attack on Kenyan territory.
No group has yet said it carried out these latest attacks, but the finger of blame will once again undoubtedly be pointed at al-Shabab or sympathisers, says the BBC's Kevin Mwachiro in Nairobi.
"We condemn this act in the strongest terms possible," Mr Ndolo said.
The Supreme Council of Kenya Muslims also condemned Sunday's church attacks, saying that "all places of worship must be respected", reported the AFP news agency.
Sunday's attacks took place during morning sermons at the churches in the garrison town.
The Provincial Medical Officer for North Eastern Province in Kenya, Mahamad Abey Shekh, said 15 people had been killed.
About 40 were thought to be wounded, several in serious condition.
The first and most serious attack took place at the AIC, police told our correspondent.
Gunmen shot two policemen outside one of the churches, and grenades were then thrown inside. As the panicked congregation rushed to escape, gunmen fired on them, police said. At least 10 people died.
In the second - apparently co-ordinated - attack at a Catholic church, two grenades were thrown inside the church. One failed to go off, but police say three people were injured by the other one.
Police said up to seven gunmen were involved in the attacks, but none had been apprehended.
Witnesses told AFP that bodies lay scattered in the blood-spattered churches as scores of wounded were rushed to hospital.
"It is a terrible scene, you can see bodies lying in the churches," regional police chief Leo Nyongesa told the agency.
"You can imagine for such a small town how the police and medical services have been stretched trying to deal with this," Mr Ndolo told Reuters news agency.
Garissa is the capital of North Eastern province, about 140km (90 miles) from the Somali border.
It is close to the Dadaab refugee camp, where gunmen kidnapped four aid workers and killed a driver on Friday in an attack Mr Ndolo said he suspected al-Shabab sympathisers of carrying out.
These two incidents have not painted a good picture of the efficacy of Kenyan security forces, our correspondent says.
Troops are supposed to have secured the Kenya-Somali border and frontier towns, but this does not seem to be happening, he adds.