Rebels battle with tribesmen over oil in Syria's east BEIRUT (Reuters) - Islamist rebels are clashing with tribesmen in eastern Syria as struggles over the region's oil facilities break out in the power vacuum left by civil war, activists said on Saturday. One dispute over a stolen oil truck in the town of Masrib in the province of Deir al-Zor, which borders Iraq, set off a battle between tribesmen and fighters from the Nusra Front, an al-Qaeda linked rebel group, which left 37 killed, according to the Syrian Observatory for Human Rights. The fighting, which started in late March and lasted 10 days, was part of a new pattern of conflict between tribal groups and the Nusra Front, said a report from the Observatory, a British-based group which opposes Syria's government and draws information from a network of activists in the country. As the civil war between President Bashar al-Assad's forces and different rebel groups enters its third year, secondary conflicts are emerging over influence and resources such as oil. More than 70,000 people have been killed in Syria's conflict so far, according to the United Nations. Rebels and local tribes in Deir al-Zor accuse each other of stealing oil from fields in Syria's most energy-rich region. Masrib tribesmen called for help from Assad's forces against Nusra, according to the Observatory and a fighter with the Islamist group. Nusra responded by blowing up 30 houses after the battle, in which 17 rebels were killed, at least four of them foreigners, the fighter said on Skype. "They (the villagers) killed some of our men and mutilated their bodies, which immediately mobilized the (Nusra) Front ... we saw that they were getting help from the regime, which sent them weapons and ammunition," he said on Skype. It was not possible to verify the reports, due to government restrictions on reporting in Syria. The incentive for disputes over lucrative resources may be increased by plans by the European Union to lift an embargo on Syrian oil, which would make it easier to sell. The EU said this week it wants to allow Syria's opposition to sell crude in an effort to tilt the balance of power towards the rebels, who are outgunned by Assad's fighter planes and long range missiles. The EU banned purchases of Syrian oil by European companies in 2011 in response to a crackdown on peaceful protests against Assad, in the first stage of what became civil war. The latest U.S. government data indicate oil production in Syria was 153,000 barrels per day in October 2012, a nearly 60 percent decline from March 2011. A YouTube video said to be from Deir al-Zor shows a tribal leader warning locals against entering conflicts over oil. The problem of all problems, the scourge of all scourges is oil. We are calling for a meeting of all villages and the countryside to discuss this issue, which could cause our annihilation and is a grave danger," he said. Reporting by Erika Solomon; Editing by Jason Webb