Hama shelling undermines Syria truce BEIRUT (Reuters) - The Syrian army killed more than 20 people in Hama on Monday, activists said, shattering a week of relative quiet in the central city visited a day earlier by U.N. monitors laying the ground for a wider mission to oversee a shaky 11-day ceasefire. A small group of unarmed observers has been in Syria for a week, tracking the truce between forces loyal to President Bashar al-Assad and opponents inspired by 'Arab Spring' uprisings in North Africa and elsewhere in the Middle East. The deal has curbed some of the violence, but the latest killings in Hama's Arbaeen district have laid bare the difficulty of bringing to a complete halt 13 months of fighting in which more than 9,000 people have died. The U.N. Security Council has approved an expansion of the monitoring mission to 300 observers, although Assad's opponents say such numbers are far too small to keep a track on events in a nation of 23 million. The U.N. political affairs chief said on Monday that the fighting in Syria was continuing despite announcements from the government that it will comply with the truce and has withdrawn troops and heavy weapons from population centers. "The cessation of armed violence remains incomplete," Lynn Pascoe, U.N. undersecretary-general for political affairs, told the Security Council during a debate on the Middle East. There was no immediate comment from Syrian authorities, who say they are committed to international mediator Kofi Annan's April 12 ceasefire agreement, but reserve the right to respond to what they say are continued attacks by "terrorist groups." A local activist called Mousab told Reuters by telephone in neighboring Lebanon that military forces entered the Hama district "and shot people in the street." It began in the morning with tanks and artillery. There were houses burning," he said, adding that at least 20 people were killed, 60 were wounded and more could be buried under collapsed buildings. The British-based Syrian Observatory for Human Rights said it had the names of 28 people killed in Hama on Monday. A group of U.N. monitors visited Hama on Sunday, and activists said soldiers opened fire to prevent a crowd from meeting them in the main square of the city, which had been relatively quiet since the ceasefire agreement. Hama is a hotbed of anti-Assad sentiment where thousands of people were killed 30 years ago in a crackdown on an armed Islamist uprising by his father, Hafez. On Monday, the nascent U.N. monitoring mission also visited Douma, a town on the outskirts of Damascus. A video provided by activists showed four blue-helmeted U.N. personnel in the middle of a throng of thousands of men chanting anti-Assad slogans. Besides calling for the arming of the rebels, the men also demanded tanks be pulled out of cities amid skepticism that Assad's forces are complying with one of the key stipulations of Annan's ceasefire deal. Douma-based activist Mohammed Doumany said security forces had simply hidden tanks and other hardware on farms around the town, beyond the prying eyes of monitors. Today everything is quiet. Yesterday and this morning there was lots of gunfire and tanks everywhere but they have hidden them to pretend to the monitors that they are respecting the ceasefire," he said. Under the terms of the deal, the U.N. monitors have to remain neutral - a stance that is drawing ire from Syrians desperate for outside help, especially after Western military intervention against Libyan leader Muammar Gaddafi. "When they went to Zabadani, they only stayed for ten minutes and refused to see anything," said Omar Hamza, an activist who lives just outside Douma, after a flying U.N. visit to Zabadani, a town near the Lebanese border. "They are only there to see if there are tanks but the government hides them," he said. The people of Zabadani are very angry. The Syrian government says 2,500 security personnel have been killed by the rebels, and the official SANA state news agency "armed terrorist" groups assassinated a colonel, two lieutenants, a warrant officer and a doctor on Sunday. Western and Arab ministers meeting in Paris last week described the observer mission as a "last chance" for peace in a state that sits at the strategic heart of the Middle East, and abuts Israel. The United States said if Damascus did not permit adequate monitoring, the Security Council should work towards imposing global sanctions that would put a far tighter economic squeeze on Assad. The European Union strengthened its own sanctions against Damascus on Monday, restricting exports of luxury goods and items that could be used for repression. Writing by Ed Cropley, editing by Diana Abdallah