rease efficiency of hydrocarbon exploration and production at the company's deposits both in Russia and abroad. "Our joint efforts are aimed at establishing a productive business relationship and adapting Schlumberger's cutting edge technology to Gazprom Neft's needs in order to achieve the best results," Gazprom Neft's Deputy Chief Executive for Exploration and Production Boris Zilbermints said. The co-operation will allow Gazprom Neft to make mhat has been seeking his ouster since early February. Al-Ahmar's armed followers fought back and reoccupied several government buildings they had seized in the first round of fighting between the sides last week. There was no immediate information on casualties. A resident of the capital, Talal Hazza, said an artillery shell exploded outside his home and another destroyed his neighbor's house. Hazza says the fighting is preventing ambulances from reaching the wounded. Beyond the capital, violence has erupted in the southern city of Taiz, which has been a hotbed of anti-government protests since the early days of the uprising against President Ali Abdullah Saleh. Soldiers backed by tanks and bulldozers have moved in to smash a tent camp in a central square in the city and also destroyed a field hospital that had been set up in anticipation of such an attack. A doctor who witnessed the attack said at least 20 people were killed on Monday. field hospital at the protest camp. Republican Guard soldiers along with security forces and armed men in civilian clothes attacked the protesters. Some set fire to dozens of tents used by the protesters, and bulldozers ran over hundreds of other tents without checking whether any protesters were still inside, two witnesses said. One of them, Mohammed al-Zarafi, said he saw tents being set on fire while injured protesters were still inside. The other witness, protester Boushra al-Maqtali, called the attack "a real massacre." "The square and the (field) hospital are in ruins," she said. "The tanks took the place of hundreds of tents that were set up there. The artillery units are occupying the whole space to make it impossible for the youth to return to the square," she said. The violence took place in Freedom Square -- a popular gathering point for thousands of anti-government protesters that have been camping there since January to demand Saleh's overthrow, Reuters reports. As police set two tents in the square on fire, protesters responded by hurling molotov bombs and rocks at them. Troops also attacked the Majeedi Hotel overlooking the square, where journalists were detained, al-Shugaa said. Then snipers took over the top of the building to shoot at protesters. Amateur video aired by Al-Jazeera TV showed masked men with rifles shooting from rooftops at the protesters in the street. Al-Shaga said "most of the injuries are critical in the head, chest and neck." He said several dozen of the injured were dragged away by security forces and vanished. The city of Taiz has been a hotbed of anti-government protests since crowds began calling for Saleh's ouster in early February. In Zinjibar, resident Ali Dahmas says he saw fighter jets firing at the southern outskirts of the town and heard loud explosions that sent up columns of smoke. He spoke to The Associated Press by phone on Monday. Military units battled the militants in Zinjibar overnight and into the morning in an attempt to clear the fighters from the town, where they've blockaded themselves behind barricades and rocks. Shelling killed at least four of the fighters, bringing the death toll there since Saturday to 34, according to an official at al-Razi hospital who spoke on condition of anonymity because he was not p