* Two churches struck by simultaneous grenade blasts
* Garissa situated near border with Somalia
* Kenya hit by series of blasts since troops sent to Somalia (Adds White House statement)
By James Macharia
NAIROBI, July 1 Masked assailants launched simultaneous gun and grenade raids on two churches in a Kenyan town on Sunday, killing at least 17 people in the worst attack in the country since Kenya sent troops into Somalia to crush al Shabaab militants.
More than 60 people were wounded in the attacks in Garissa, the north Kenya town which has been used as a base for operations against al Qaeda-linked insurgents in Somalia.
"This is the worst single attack since October, when our troops went into Somalia," national police spokesman Eric Kiraithe told Reuters.
"It is the worst in terms of the numbers killed, the manner of execution, the anger behind it and the anguish it has aroused as well as the national impact it has had."
There was no immediate claim of responsibility for the attacks in Garissa, a largely Muslim town of 150,000 with a significant ethnic Somali population.
Police said they suspected al Shabaab sympathisers or bandits may have been behind the raids, but it was too early to say. In Somalia, al Shabaab declined to comment.
White House spokesman Jay Carney said the United States "strongly condemned" the attacks at a time of transition in the country. Kenya is due to hold an election next March.
Regional deputy police chief Philip Ndolo told Reuters from Garissa that seven attackers had hurled grenades into the Catholic Church and the African Inland Church (AIC) and then opened fire with assault rifles.
They struck the churches, situated 3 km (two miles) apart, at around 10.15 a.m. (0715 GMT). Two policemen guarding the AIC church following previous attacks were among those killed.
"The goons were clad in balaclavas," said Ndolo.
"You can imagine for such a small town how the police and medical services have been stretched trying to deal with this."
Television footage showed benches knocked over at the AIC church and blood pooled on the floor and spattered over the walls. Garments, shoes and bibles were strewn around.
Police milled outside the churches which were cordoned off by investigators who were picking at fragments and taking notes.
"We have 17 bodies at the mortuary so far," regional medical officer Abdikadir Sheikh told Reuters.
Paul Mwalali, 52, a worshipper at the AIC church, told the Daily Nation newspaper he heard objects hit the roof before explosions rocked the church.
"I had a front row seat in the church. I heard something fall on the roof. Then there was a huge explosion. I (fell) on the ground. Then there was shooting and people were screaming," he said.
Felix Kimanzi told the newspaper he saw masked gunmen hurl two grenades, but only one exploded.
"I was 100 metres away from the church when I saw two gunmen at the entrance spray bullets at the congregation," he said.
"They were joined by two more gunmen in blue uniforms who hurled grenades and they all fled on foot."
Seven people who were badly wounded in the attacks were airlifted to Nairobi from Garissa.
The attacks were the latest on Christian worshippers in Kenya after two people were killed in grenade blasts in March and April in Nairobi and Mombasa.
The latest coordinated assault resembled the tactics of Nigeria's Islamist militant group Boko Haram, which has killed hundreds of people on the other side of the continent since the movement started its uprising more than two years ago.
Vice President Kalonzo Musyoka condemned the Garissa attacks and said Kenya would not be intimidated by such "cowardly acts" aimed at instilling fear.
The Supreme Council of Kenya Muslims condemned the assault, saying "all places of worship must be respected."
Garissa, a market centre for the trade in camels, donkeys, goats and cattle, is about 100 km (60 miles) from Kenya's Dadaab refugee camp, where gunmen kidnapped four aid workers and killed a driver on Friday before fleeing towards the border with Somalia.
Kenyan forces thrust into Somalia after raids in the border region and kidnappings that threatened the tourism industry in the region's biggest economy and wider regional destabilisation.
Last Sunday, three people were killed in a grenade attack at a night club in the port city of Mombasa, a day after the U.S. embassy in Kenya warned of an imminent attack on the city.
(Additional reporting by Noor Ali and Feisal Omar; Editing by Sophie Hares)