"This is a very serious development, and the situation remains very delicate," he said.
"Saada is a volatile region, cordoned off by the army - it is essentially off limits, even to local media."
Another Al Jazeera correspondent said that the Imam of the mosque was the most likely target of the blast. He is also a military leader in the area.
Saada province, situated in northwest Yemen, has been the site of a rebellion by members of the Zaidi community, a branch of Shia Islam, that has killed thousands of people since 2004.
"We criticise and condemn this regrettable incident"
Abdul-Malik al-Huthi, rebel leader
The fighters are known as Huthis, named after Hussein Badr Eddin al-Huthi, their former commander, who was killed by the army in September 2004.
The government blamed the Huthis for the attack and said six people had been arrested.
"Those who carried out this ugly crime are terrorists and criminals linked to the terrorist Abdul-Malik al-Huthi," an interior ministry statement said, referring to the brother of the late commander.
However, the group has denied any responsibility for Friday's attack.
Abdul-Malik al-Huthi told Al Jazeera: "We condemn this regrettable incident and deny categorically any role in this incident.
"It is not part of our ethics to target mosques or worshippers at all."
The renewed violence comes despite efforts to implement a peace deal between the government and the fighters, brokered by Qatar in June 2007.
The agreement, under which the fighters would lay down their arms, was revived during a meeting between the two sides in Doha in February.
In January, clashes renewed between rebels and thousands of government troops backed by tanks, artillery and helicopters. Tribal leaders in the northern region say more than 30,000 residents have been displaced by the fighting.
The Huthis have been fighting to restore the Zaidi imamate, which was overthrown in a coup in 1962.
They reject Ali Abdullah Saleh, the president, and his government although Saleh himself is a Zaidi.