"(The authorities) are working to free the three Germans and arrest the kidnappers and bring them to justice," said regional security chief, Colonel Saleh Dahmesh.
A tribal source said police had arrested at least 100 tribesmen linked to the kidnappers in a bid to pressure them to release the captives, who were seized on Sunday and are being held in the Bani Dhabyan area about 80 kilometres (50 miles) east of Sanaa.
The source said one of the kidnappers was demanding $200,000 to recompense him for lost land and that police release his brother and son who were arrested four months ago over a land dispute.
Dahmesh was quoted as saying by the Yemeni defence ministry's online newspaper that police suspected a local tribal leader named Abd Rabbo Saleh al-Tam of masterminding the kidnapping.
A German Foreign Ministry spokeswoman in Berlin said a crisis team was still working on the case. "They are in close contact with the Yemeni authorities and working for a
peaceful resolution," she said, declining further comment.
A Western diplomat told AFP that the German embassy had asked the Yemeni authorities not to resort to force to free its nationals.
"Tribal leaders, some of them members of parliament, are acting as go-betweens for the kidnappers and have contacted the German embassy in Sanaa to reassure them that the three hostages remain in good health," a tribal source said. "The kidnappers are sticking to their demands... and have sent a message to the German authorities through the embassy in which they say that they are victims of an unjust government."
Yemen, the ancestral homeland of al-Qaeda chief Osama bin Laden and one of
the poorest countries on the planet, is a strongly tribal country and awash
Its powerful tribes have abducted more than 200 foreigners over the past 15
years in a bid to extract concessions from the central government whose writ
extends with difficulty over the lawless countryside.
A Western diplomat said the hostages were a German couple and their daughter, who works in Yemen for German development group GTZ, which employs some 12,000 staff in more than 120 countries.
The pan-Arab newspaper Asharq Al-Awsat said the daughter had lived in Yemen for more than a decade and that her parents had visited her in the country several times in the past.
All foreign hostages have been freed unharmed except for three Britons and an Australian seized by Islamist militants in December 1998. They were killed when security forces stormed the kidnappers' hideout.
In December 2005, armed Yemeni tribesmen captured a former German ambassador and foreign ministry number two, his wife and three children and held them for about five days.