SANAA (AFP) - More than 80 civilians were killed in an air raid which blasted a makeshift camp of displaced people in northern Yemen, witnesses said on Thursday, as the army pursued its offensive on Shia rebels. One witness, reached by telephone, told AFP that most of those killed in Wednesdays raid were women and children. The attack was carried out by a warplane (that) targeted displaced families who had gathered under trees in the area of Adi, in Amran province - scene of heavy fighting between the army and the rebels, the witness said, asking not to be identified. Another witness, also reached by telephone, told AFP that at least 87 were killed in the attack, which was acknowledged by a Yemeni official. The jet fighter targeted Huthi (rebels) who were firing (while hiding) among the displaced people, the official told AFP requesting anonymity. He declined to comment on the death toll. A rebel statement condemned the attack, accusing the Sanaa government, which has vowed to crush the five-year-old rebellion, of thirsting for blood. The bloodthirsty authorities have committed a new massacre, said a statement issued by the Huthi rebels. It said that government MiG warplanes at 12:00 noon (0900 GMT) on Wednesday had targeted displaced people gathering along the Barata road, close to Adi village near Harf Sufyan, which lies on the route linking Saada to the capital. Dozens were killed and the bodies were blown away by the impact of the strike, the statement said. The Yemeni army, which launched operation Scorched Earth against the rebels on August 11, said Thursday it has delivered heavy blows over the past hours. A military commander claimed the army had killed and wounded many rebels, whom it accused of using civilians as human shields. He did not elaborate. New York-based Human Rights Watch urged the Yemeni government to promptly and impartially investigate responsibility for any attacks on civilians. In a statement it also urged all parties to the armed conflict in the region to respect the prohibition under international law against targeting civilians. There has been no reliable sources for the death toll of the ongoing fighting. The government accuses the rebels of seeking to restore the Zaidi Shia imamate which was overthrown in a 1962 coup that sparked eight years of civil war. An offshoot of Shia, the Zaidis are a minority in mainly Sunni Yemen but form the majority community in the north. The Zaidi rebels are known as Huthis, named after their late leader Hussein Badr Eddin al-Huthi, who was killed by the army in September 2004 Relief groups have warned of worsening humanitarian conditions among the tens of thousands of civilians forced from their homes by the latest fighting. The United Nations and the international Red Cross on Tuesday both called for an aid corridor to allow urgent relief supplies to reach civilians. The rebels said on Tuesday they were ready for an unconditional ceasefire in a letter addressed to UN Secretary General Ban Ki-moon, a copy of which was received by AFP. But the government has insisted that the rebels should fulfil six conditions it had stipulated in an earlier truce offer, including a full disarmament of the rebels, whom it has repeatedly accused of being backed by Shia Iran.