Yemeni government troops were locked in heavy fighting on Thursday with Shiite rebels in northern Yemen where at least 50 people have been killed in the past two days, military and tribal sources said. Dozens more may have died in the provincial capital of Saada, where army officials and eyewitnesses said street battles intensified late on Thursday. Some 21 rebels and four soldiers, including a colonel, were killed in the Harf Sufyan area of Amran province, where clashes broke out Wednesday night and continued on Thursday, the military and tribal sources told selfnews by telephone. Witnesses said that Yemeni warplanes engaged in action to back up troops around Harf Sufyan as well as in Dhahran, Anad and Al-Aqab districts of the Saada province, stronghold of the Zaidi rebels, also known as Huthis. Army helicopter gunships opened fire on positions near Samaa, in Saada city's southern suburbs as rebels clashed with troops in the area, the witnesses said. Another 23 rebels were killed in sporadic clashes in Dammaj southeast of Saada, tribal sources and witnesses told selfnews, adding that the fighting there pitted Shiite rebels against Sunni fighters backed by tribe members. Two rebel leaders identified as Farhan Maqit and Hassan Thawrat died in clashes with between police and members of a local tribe in Baqem and Munabbeh villages near the border with Saudi Arabia, tribal sources said. Witnesses said that Maqit and Thawrat -- who was said to have played a key role in previous attempts to reach a ceasefire between rebels and government troops -- lost their lives in an air raid on their hideout in Baqem. In Saada city, violent clashes erupted in streets around the Palace of the Republic, a government administrative building, and in other neighbourhoods, witnesses and military officials reported. The skirmishes caused 'dozens of deaths on both sides,' the sources said, without being able to provide details. Reports of the deaths could not be confirmed from the rebel side as the city is cut off. The Yemeni military launched 'Operation Scorched Earth' on August 11 aimed at eradicating the rebellion in the mountainous north. Two separate ceasefires have lasted only a few hours before fighting erupted again. The government accuses the rebels of being backed by Iran. The rebels deny the claim and allege Sanaa has brought in Saudi warplanes to support the army. Yemeni rebels said on Tuesday they want an independently monitored halt to the government's offensive while the authorities have demanded that the rebels agree to a set of conditions first. The main government demand is that the rebels 'respect the ceasefire and the opening of roads, evacuate their positions and free captured civilians and soldiers.' The government says the rebels want to restore the Zaidi Shiite imamate, a form of clerical rule that was overthrown in a republican coup in 1962 that sparked eight years of civil war. An offshoot of Shiite Islam, the Zaidis are a minority in mainly Sunni Yemen but form the majority community in the north. President Ali Abdullah Saleh is himself a Zaidi. 