Jarallah is one of two suspected militants being tried for murder
A suspected Islamic militant has gone on trial in Yemen, charged with murdering three American Christian missionaries last December.
The two women and one man were shot dead at the Southern Baptist missionary hospital in the southern town of Jibla.
Abed Abdulrazzak Kamel told the court he had carried out the killings.
He said he had co-ordinated the attack with Ali al-Jarallah, another suspected militant, whose trial over a separate murder also opened on Sunday.
Prosecutors say Mr Kamel, 30, smuggled a rifle into Jibla's Baptist hospital and shot dead a doctor and her two colleagues, also wounding an American pharmacist.
I killed them to take revenge on Christians and Americans
He faces the death penalty if convicted.
As Mr Kamel's trial opened in the town of Ebb, about 200 kilometres (125 miles) south of the capital, Sanaa, he said he felt "comfortable" about the murders.
"I killed them to take revenge on Christians and Americans," he said.
Mr Kamel told the court he had carried out the attack in co-ordination with Ali al-Jarallah.
Mr Jarallah is on trial in Sanaa for the separate killing of Jarallah Omar, deputy leader of the Yemeni Socialist Party, at a party conference in Sanaa in December.
Martha Myers, 54, from Alabama - killed
William Koehn, 60, from Texas - killed
Kathleen Gariety, 53, from Wisconsin - killed
Donald Caswell, 49, from Texas - seriously injured
He is accused of premeditated murder and of forming a ring whose aim was to murder politicians, journalists and others.
Mr Jarallah, a former member of Yemen's Islamic opposition, is said to have been highly critical of moderates within the party and the government.
Yemen has been a focus for anti-Western attacks in recent years.
In October 2000, 17 US sailors died in a suicide attack on the destroyer USS Cole in the southern port of Aden.
Since the 11 September 2001 attacks in the United States, Yemen has been cracking down on Islamic militants members in an attempt to shed its image as a haven for al-Qaeda supporters.