At least 16 suspected rebel fighters and two Yemeni soldiers have been killed as heavy fighting in the north of Yemen entered its fourth day, according to officials.
Thousands of people in Saada province have been forced from their homes as the Yemeni military has bombarded suspected bases of the Shia Muslim Zaydi sect in recent days.
Local officials said on Friday that the fighting had shifted from Saada town towards the southern border of the province, just 70km north of the capital, Sanaa, and a government building in Amran province had come under attack.
"Over the past four days, 17,000 families have been forced to flee their homes," Hassan Mohammed Manaa, the Saada provincial governor, said.
He said that the fighters had killed four leaders of the Al-Azl tribe and 15 other civilians, including women and children, during the violence.
Scores of people are thought to have been killed or wounded in the region in recent days.
Aid workers 'seized'
The latest clashes came after at least 15 Red Crescent aid workers were reportedly taken from a refugee camp on Thursday.
"They blindfolded them, beat them and insulted them," the September 26
newspaper, published by the defence ministry, quoted Manaa as saying.
A number of doctors and nurses, as well as officials and administrators, were among those taken from the camp, Manna said.
The Yemeni government has vowed to hit the fighters, followers of Abdul-Malik al-Houthi, with an "iron fist" unless they meet its conditions for a ceasefire.
Under the terms announced by the country's supreme security committee, headed by Ali Abdullah Saleh, the president, the fighters were told they must evacuate all occupied government offices, hand in ammunition and equipment and free prisoners.
The committee said the authorities would stop the military offensive and release all detainees seized on suspicion of having links to the Houthis if they complied, the official Saba news agency reported on Thursday.
'Crimes against humanity'
However, al-Houthi rejected the conditions in a statement, saying the government was seeking to "mislead the public opinion".
"Authorities are not actually seeking a radical settlement for this issue ... and their conditions are merely intended to mislead the public opinion after the crimes against humanity they committed in Saada villages."
He called on the government to comply with a ceasefire agreement signed by the two parties in Doha, the Qatari capital, in June 2007.
Tensions have been rising between the Houthis and the army in Saada since last July when Saleh declared the conflict over.
Fighting between the Houthis and the military has left hundreds of soldiers and fighters dead since 2004.
The authorities have accused the Houthis of trying to reinstall the rule of imams, toppled by a republican revolution in northern Yemen in 1962. The fighters say they are defending their villages against government oppression.