The Yemeni army have been fighting a Shia insurgency
More than 80 people have been killed in an air raid on a camp for displaced people in northern Yemen, reports say.
According to witnesses, many of those killed in the raid - which took place near the border with Saudi Arabia - were women, children and old people.
Government forces have been trying to contain a growing insurgency in the area by rebels known as Houthis.
Fighting has intensified since the Yemeni army launched an operation targeting the rebels in mid-August.
The UN estimates that this latest wave of fighting has added up to 50,000 people to the existing 100,000 made homeless by earlier rounds of fighting.
Local people said a large group of refugees who had gathered on the hillside attracted the attention of more than one government plane.
One witness told AFP news agency that the attack happened as displaced families gathered beneath trees at Adi in the Harf Sufyan area, in Amran province.
Another witness who did not want to be named said: "The camp was taken by surprise by the air force bombing them. When one plane starting firing some people ran towards the water canal, but they were killed when the plane fired at them again."
Local leader Sheik Mohammed Hassan, attending a mass funeral for the victims, said the situation was "horrendous".
"Whoever did this must be held accountable," he said.
The UN's refugee agency, UNHCR, said in a statement it was "alarmed" by reports of the attack, which it said underlined the need to create humanitarian corridors in northern Yemen to allow aid to reach displaced people.
Yemeni officials deny that there was a refugee camp in the area and claim that the army had only struck rebels and their supply lines.
An official said that the rebels had been shooting from the area ahead of the attack, which reports say happened on Wednesday.
"The jet fighter targeted rebels who were firing while hiding among the displaced people," said the official, who spoke to the AFP news agency on condition of anonymity.
The al-Haq Shia opposition party condemned the attack, describing it as part of "recurring massacres against the people of Saada and Harf Sufyan" and demanded an investigation into the air raid.
The New York-based group, Human Rights Watch, has called on the Yemeni government to investigate the reported attack on displaced people.
Houthi rebels say they want greater autonomy and a greater role for their version of Shia Islam.
They complain that their community is discriminated against and that the most recent attack is only one example of heavy civilian casualties from aerial bombing.
The government accuses them of seeking to overthrow it to impose Shia religious law and of having links to Iran.
The BBC's Bob Trevelyan says that both sides see unwelcome influences from abroad, with government accused of being influenced by Sunni-dominated Saudi Arabia.
The rebel fighters - drawn from the country's Zaidi Shia minority - are followers of Shia cleric, Hussein al-Houthi, who was killed in 2004.
Tens of thousands of Yemenis have fled the fighting in the north, cramming into makeshift camps, schools and barns, as aid groups struggle to get supplies to them.
The UN said that the plight of civilians has reached "alarming levels" and that there has so far been no response to its appeal to raise US $23.5m to help the displaced.
The Yemeni government is also battling secessionists in the south and has been criticised by the US for its failure to tackle al-Qaeda militants in the east and pirates off the coast.
The United States has signed a new aid deal, promising the Yemeni government a further US $120m.