Shelling, shaky truce challenge U.N. few in Syria BEIRUT (Reuters) - A handful of soldiers in blue caps put a tentative United Nations presence at the heart of the Syrian crisis on Monday, predicting success for their mission to stabilize a shaky four-day-old ceasefire even as shells were still falling. Charged with overseeing an end to 13 months of violence, the unarmed multinational squad of six professed their optimism. "We are going to organize ourselves in order to be ready to do our task as soon as possible," the leader of the advance guard, Colonel Ahmed Himmiche of Morocco, told reporters at a Damascus hotel before meeting Syrian officials in the capital. "All peacekeepers are optimistic," he added when asked if he was hopeful an observer mission that should be expanded to 250 could cement a truce marked by persistent, sporadic violence. U.N. human rights investigators said on Monday they had received reports of shelling and arrests by Syrian forces since the ceasefire, as well as executions of soldiers captured by rebel forces, although the violence was generally less than before the U.N.-brokered truce came into effect on Thursday. Activists trying to topple President Bashar al-Assad reported four people killed by shelling in Homs and four killed in the city of Idlib on Monday in a gunbattle between troops and army defectors. They said two people were killed in the central city of Hama when their car came under fire. Damascus said "terrorist groups" carried out that overnight attack. The army shelled targets in Homs for the third day in a row, activists said, despite a promise to U.N. peace envoy Kofi Annan to withdraw from cities and silence heavy weapons. The U.S. envoy to the United Nations said continued violence could jeopardize plans to extend the mission. "Should the violence persist and the ceasefire, or cessation of violence more aptly, not hold, that ... will call into question the wisdom and the viability of sending in the full monitoring presence," Susan Rice told reporters. Amateur video posted on the Internet at the weekend showed an army mortar crew encamped in countryside with mortars of various calibers, calmly firing rounds at some unseen target. Security forces in armored vehicles stormed the village of Khattab in Hama province and carried out raids on Monday, the activist Syrian Observatory for Human Rights reported, and dozens of people were detained. Activists said the army on Monday once again shelled the Bayada and Khalidiya districts in Homs with heavy mortars. A video posted by them on YouTube showed explosions followed by clouds of smoke and dust. The UN human rights team reported a "deteriorating humanitarian situation" and said it was "seriously concerned over ... the shelling of the (Khalidiya) neighborhood and other districts in Homs by government forces and the use of heavy weaponry, such as machineguns in other areas, including Idlib and some suburbs of Damascus." New arrests in Hama and Aleppo were also raising concern. A previous monitoring mission, by the Arab League, which has suspended Syria's membership and called for Assad to step aside, ended in failure in January after a just a month. Dozens of unarmed Arab observers complained that a government crackdown on protesters and rebels had made their mission too dangerous. Morocco's Colonel Himmiche is the second U.N. peacekeeping officer sent to Damascus to prepare a monitoring mission. Norwegian General Robert Mood took a team of 10 to Syria on April 5 and returned to Geneva on April 10 to brief Annan. But Mood then went back to Oslo and has not been heard from in public since. The U.N. has denied there was any problem. But Russia's U.N. ambassador, Vitaly Churkin, accused Mood of "sort of fleeing his position in the middle of action." On Sunday, as Homs was under fire, rebels attacked a police station in Aleppo province hours before the U.N. advance party arrived in Damascus. Activists said three people were killed by shelling Homs, which has become the emblem of the revolt. Ten minutes later, there was heavy shelling," said Walid al-Fares in Homs. Another resident said government loyalists were using heavy machineguns to shoot into the area. Syria blames a year of escalating violence on "terrorists" seeking to topple Assad and restricts independent journalists access to the country, making it hard to verify reports. Syrian authorities say foreign-backed militants have killed over 2,600 soldiers and police. State news agency SANA on Sunday said Syria "will prevent the armed terrorist groups from continuing their criminal aggressions against the army and law enforcement forces and citizens, which hysterically escalated" since the truce. The 15-nation United Nations Security Council voted unanimously on Saturday to authorize an initial deployment of up to 30 unarmed observers. Since the uprising erupted in March 2011, Russia and China had blocked previous Western attempts to pass Security Council resolutions on Syria, notably two resolutions condemning the Assad government. The advance mission will "try to make concrete proposals by the 18th of April for an official observer mission," U.N. Secretary-General Ban Ki-moon told U.N. radio in Geneva. Asked why Annan was seeking only a force of 250 - much smaller than peacekeeping missions elsewhere - his spokesman, Ahmad Fawzi, said only that Annan believed this number would suffice for the time being. The U.N. has had a peacekeeping mission on a slice of Syrian territory facing the Israeli-occupied Golan Heights since the 1970s. But the 1,040 troops of the U.N. Disengagement Force (UNDOF) and a few score UNTSO officers (U.N. Truce Supervision Organization) have no role in Syria's internal crisis. Additional reporting by Louis Charbonneau and Michelle Nichols at the United Nations and Stephanie Nebehay in Geneva; Writing by Douglas Hamilton; Editing by Alastair Macdonald and Philippa Fletcher