One person died and 15 people were wounded when a grenade was thrown into a church in Kenya's capital during Sunday service, an official said.

Nairobi's deputy police chief, Moses Ombati, said the grenade exploded at God's House of Miracles International Church in Nairobi.

Doctors at Nairobi's Kenyatta Hospital said they had treated 11 patients wounded in the attack. None of the injuries were life-threatening, said a doctor who refused to give his name because he was not authorized to speak to reporters.

Steven Mulinge, a church pianist who was among the wounded, recalled seeing a man walking through the entrance and hurling what looked like a stone at the altar.

"I heard a blast and then around me everyone was covered in blood," he said. "Later I found myself lying down with a deep cut on my hip."

He said about 50 people were attending the service when the attack happened.

The incident is the latest in a string of grenade attacks since Kenya sent troops into Somalia in October last year.

In March, grenade explosions at one of the main bus stations in Kenya's capital killed nine people and wounded 40 others, the deadliest in the series of attacks.

Al-Qaida-linked al-Shabab militants from neighboring Somalia have vowed to carry out a major attack on Kenya for sending troops in.

Sunday's attack follows a U.S. Embassy warning that a terror attack on prominent government buildings and hotels in Nairobi could be imminent.

The embassy said Monday that the timing of the attack is not known but they believe it to be at a final planning stage.

Police have said that the Somali militant group is suspected of killing at least 30 Kenyan civilians since Kenyan troops entered Somalia.

In February, police spokesman Eric Kiraithe said the killings -- which go back to October -- are believed to have been carried out by sympathizers of al-Shabab in Kenya.

He said most of the attacks were carried out in towns near the border between Somalia and Kenya.

Police say that dozens of Kenyan youth have been recruited by al-Shabab and are operating in the country. Al-Qaida announced in February that it was merging with al-Shabab.

Some church members said they suspected the attack was related to an unresolved land dispute.

James Maina, who described himself as a church elder, said his church may have been targeted by neighbors who claim to own the land on which it is built. He said the church had suffered minor attacks in the past, including having its windows smashed.

"This thing is between the mechanics and the church," he said, referring to the motor repairmen who he said want them gone. "The mechanics don't want us to utilize this land. They are against us."

Ndegwa Muhoro, Kenya's director of criminal investigations, told reporters outside the church it was "too early to say" who might be responsible.

"We need to maintain vigilance because we don't know who they are," he said.