SANAA (Reuters) - Yemeni Shi’ite rebels accused the U.S. air force on Tuesday of joining attacks against them, and killing at least 120 people in a raid in the north of the poor Arab state.

“The savage crime committed by the U.S. air force shows the real face of the United States,” said the northern rebels, who often report attacks by the Yemeni and Saudi fighter planes, on their website. There was no immediate report of U.S. comment on the alleged incident.

The rebels, who are fighting the Yemeni army and forces of neighboring Saudi Arabia, posted videos on the Internet that appeared to show people trying to clear rubble covering human bodies.

On Sunday, the rebels said at least 70 people had been killed in a Saudi air raid on a market in the northern town of Razeh. The reports could not be verified as aid workers and media have limited access to the conflict zones.

U.S.-allied Saudi Arabia, the world’s largest oil exporter, fears the growing instability in neighboring Yemen could turn into a major security threat for the kingdom by allowing al Qaeda to gain a stronger foothold there.

In Geneva, a U.N. official said U.N. agencies had appealed for $177 million to help people affected by the conflict.

“The humanitarian situation is deteriorating, notably for 200,000 people displaced by successive conflicts since 2004,” Elisabeth Byrs of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told reporters in Geneva.

“We demand full and unlimited access to these displaced populations as well as a halt to attacks on humanitarian convoys,” Byrs said.

U.S.-based Human Rights Watch (HRW) accused Yemeni security forces of widespread abuses and the killing of at least 11 protesters in response to calls for secession in southern Yemen.

“Yemeni authorities are violating basic rights in the name of maintaining national unity,” said Joe Stork, deputy director at Human Rights Watch’s Middle East division.

“Southern Yemenis should have the right to peacefully assemble and express their opinions, even on critical issues like secession,” he said.

“On six occasions during 2008 and 2009 ... security forces opened fire on unarmed protesters, often without warning and aiming at them from short range. At least 11 people were killed and dozens were wounded,” HRW said in a report on Yemen.

Activists, some of whom belong to the Southern Movement, have stepped up demonstrations in the past year, complaining that the government and northerners exploit and discriminate against the south, which holds most of Yemen’s oil facilities.

Yemen’s state news agency Saba said Information Minister Hasan al-Lawzi met an HRW delegation on Tuesday in Sanaa and said, “Yemen is a democratic country which supports press freedom and respects human rights.”