More than 100,000 people have been displaced since 2004
Violent clashes have broken out between Shia rebels and government forces in northern Yemen, with both sides accusing the other of breaking a truce.
A Yemeni military source claims there were many casualties, although there is no independent confirmation.
The truce had been agreed to allow aid agencies to help tens of thousands of people known to have fled their homes.
The rebels allege government persecution, while Yemeni officials say rebels want to take over the country.
Hmoud Abbad, Yemen's minister of youth affairs, speaking to al-Arabiya Television, blamed the rebels for breaking the truce.
"Those insurgents and terrorists cannot commit to any deal," Mr Abbad reportedly said.
He added that the government and armed forces had a "responsibility to put an end to those terrorists... and destroy this insurgency."
His claim was dismissed by the rebels, who accused the government forces of fighting.
Their spokesman, Mohammed Abdel-Salam, said the government was "making up excuses to keep the conflict going," Associated Press reported.
The Shia rebellion against the Sunni government began in 2004.
In the past three weeks about 35,000 people have been displaced by fighting in Saada province, according to United Nations's estimates.
Earlier this week Yemeni authorities rejected an offer of a truce from one rebel group, the Houthis, which it accuses of trying to spread a form of Shia fundamentalism.
In response the rebels originally promised to continue fighting, but as of Friday it seemed a truce had been agreed.
One rebel spokesman had told journalists the truce would be adhered to for the sake of civilians fleeing their homes.
In reality, the truce collapsed a few hours after coming into force.
A senior security commission spokesman told reporters the rebels had broken the ceasefire in the Malaheez and Hafr Sufyan regions.
More than 25,000 displaced people have been registered by the International Committee for the Red Cross and the Yemen Red Crescent Society since a new government offensive - Operation Scorched Earth - began in August.
"The dire humanitarian situation is hitting women and children especially hard," said Daniel Gagnon of the Red Cross speaking from Yemen.
The International Committee of the Red Cross, which is working with the Yemeni Red Crescent Society in Saada, said the humanitarian situation was steadily deteriorating.
It has stressed that it needs unconditional access to civilians in order to help them.
UN humanitarian officials have launched a $23.5m (£14.3m) appeal.
The latest clashes follow five years of intermittent fighting between the government and the rebels.
The rebels say a corrupt Yemeni government, backed by the West and Saudi Arabia, is using security concerns as a pretext for persecuting their community and trying to suppress their version of Shia Islam, known as Zaydism.