Masked gunmen killed 17 people and wounded dozens in gun and grenade attacks on two churches Sunday in the Kenyan town of Garissa near the border with Somalia, officials said.
As Kenya vowed not to be deterred, Washington extended its support to the east African powerhouse and the Vatican expressed concern that attacks on Christian worshippers were spreading.
Wearing balaclavas, the attackers killed two armed police -- posted outside churches following previous attacks -- before bursting inside to target worshippers as they held prayer services.
Witnesses said bodies lay scattered on the floor inside the blood spattered buildings -- a Roman Catholic church and an Africa Inland Church.
"It is a terrible scene, you can see bodies lying in the churches," said regional police chief Leo Nyongesa.
At least 40 people were rushed to hospital, several in a critical condition, the Kenyan Red Cross said.
The Red Cross, which put the death toll at 17, flew the three most critically injured victims by air ambulance to the capital Nairobi.
Three children were reported to be among the dead.
"It is a horrible sight to see," said Hussein Abdi, a resident of Garissa, a garrison town populated mainly by Muslim ethnic Somalis and located some 140 kilometres (85 miles) from the border.
Condemning the attacks as a "horrible and very worrying act", the Vatican deplored that "among terrorist groups, attacks on Christians gathered on Sunday in their place of worship has become a method, believed to be particularly efficient, of spreading hatred and fear," spokesman Federico Lombardi said.
Nigeria has suffered a string of almost weekly attacks on church services in recent months in which dozens of faithful have been killed.
"The perpetrators of these attacks have shown no respect for human life and dignity, and must be brought to justice for these heinous acts," White House spokesman Jay Carney said.
Canada also condemned the "vicious and cowardly" church attacks.
Kenya has suffered a spate of gun, grenade and bomb attacks since sending troops into southern Somalia last October to target Shebab rebels fighting to overthrow the weak UN-backed government in the Horn of Africa state.
Later, the pro-Shebab Twitter site Al-Kataib boasted of a "successful operation in Garissa" but did not specifically claim responsibility. Nairobi has blamed previous assaults on the Al-Qaeda linked Shebab.
Kenyan Vice President Kalonzo Musyoka said Sunday that the nation "will not be intimidated by such cowardly acts" and urged Kenyans to "exercise religious tolerance."
The Supreme Council of Kenya Muslims condemned the attacks, saying that "all places of worship must be respected."
"We want to send our condolences, and we are sad that no arrests have been made yet," said chairman Abdulghafur El-Busaidy.
Police said up to seven men had carried out the attacks which came two days after a gunman killed a Kenyan driver and abducted four foreign aid workers from the Dadaab refugee camp, some 80 kilometres (50 miles) to the northeast.
Last week a grenade blast in a bar in Kenya's main port of Mombasa killed three people, a day after the US embassy warned of the threat of attack.
Search efforts continued for the abducted aid workers, two men and two women who work with the Norwegian Refugee Council (NRC), and who come from Canada, Norway, Pakistan and the Philippines, according to police.
Attacks and cross-border raids in the region blamed on the Shebab, including the kidnapping in October of two Spaniards working for Medecins sans Frontieres (Doctors Without Borders), were key to Kenya's decision to invade Somalia.
The Shebab still control large parts of southern Somalia, despite African Union troops, government forces and Ethiopian soldiers wresting control of several key bases from the insurgents.
Since the 1991 ouster of then president Mohamed Siad Barre, Somalia has been governed by ruthless warlords, rival militia groups, pirate gangs and Islamist fighters, each controlling their own limited fiefdoms.
Hundreds of thousands of Somalis have fled to Kenya as well as other neighbouring nations since the collapse of a formal government two decades ago, while crippling drought and famine racked the lawless nation last year.