Rebels go on 'offensive' as attacks by Syrian regime spread NEW: 7 people were killed Monday, an opposition group says NEW: 2,000 refugees have crossed into Lebanon since Sunday, U.N. refugee agency says NEW: Explosions and gunfire were heard Monday in part of Homs, an opposition group says A Free Syrian Army official says the regime is changing tactics because of defections (CNN) -- After pummeling the western city of Rastan over the weekend, government troops turned their lethal attention to cities across the country Monday, opposition activists said. At least seven people were killed, including three in Daraa, one in Aleppo, two in Idlib, and one in Hama, said the Syrian Network for Human Rights, an opposition activist group. The Syrian regime has also ramped up raids and arrests across the country, detaining hundreds of civilians in the past two days, the network said Monday. It said Syrian journalist and blogger Rafaa Masri was among those recently detained. The Syrian government, meanwhile, said 12 "martyrs" from the army were buried Monday. As many as 2,000 refugees have crossed into Lebanon since Sunday, according to Dana Suleiman, spokesman for the United Nations' refugee agency UNHCR in Beirut. They came from Homs province, she said. Explosions and gunfire could be heard Monday in the Jib Jandali neighborhood of Homs province, according to the opposition Syrian Observatory for Human Rights. Rebel forces said they drove out the army in Rastan -- but also ceded that most of their fighters had retreated from the besieged area, which sits between the flashpoint cities of Homs and Hama. Government forces pounded Rastan with 15 rockets in 15 minutes Sunday, killing three people and wounding dozens more, most of them children, opposition activists said. Graphic video posted on YouTube shows three girls, including a 1-year-old, suffering from severe injuries purportedly sustained in that attack. Capt. Ammar al-Wawi said the Free Syrian Army's withdrawal from Rastan "was strategic to save the people's lives." "We don't want to give the regime any excuse to kill more civilians," Wawi said Monday. It was a tactical withdrawal in order to create better circumstances and to get ready for the next step. Though they are outnumbered and out-armed by the Syrian military, members of the Free Syrian Army managed to attack an air force intelligence building in Harasta, near Damascus, with machine guns Sunday night, FSA deputy head Malek al-Kurdi said. "The operation in Harasta is part of the response to the massacres of the regime against our unarmed people," Wawi said. After the regime committed massacres in Homs and other Syrian cities -- in addition to targeting the journalists and the civilians in the country -- the Free Syrian Army moved our operations to the offensive phase in defense of the Syrian people. Wawi said he thinks a growing number of defections from Syrian troops are affecting the government's tactics. "The regime is avoiding direct confrontations with the FSA fighters, so they attack and bomb the cities using artillery ... and rockets because when they fight us on the ground, we always end up getting more defectors joining our sides," he said. Meanwhile, residents in the devastated Baba Amr neighborhood of Homs endured another day with scarce or no access to running water, electricity and medical supplies, as the humanitarian toll of the nearly year-old Syrian conflict escalates. Mardini said the ICRC and the Syrian Red Crescent were delivering food and hygiene kits in two neighborhoods adjacent to Baba Amr -- al-Tawzee and al-Inshaat. "We were supposed to be there yesterday but we were not allowed," Mardini said. A convoy of aid materials arrived today to Homs from Damascus and it contains food supplies to cover the needs of several thousand people. According to SANA, the state-run news agency, authorities were busy Sunday "removing the destruction and debris left by the armed terrorist groups" in the Baba Amr and Inshaat neighborhoods of Homs." The remains of Marie Colvin, a U.S. journalist who was killed in the conflict, will arrive in the United States on Tuesday, the U.S. Embassy in Paris said. Syria's crackdown on civilians seeking al-Assad's ouster has drawn widespread criticism from the international community. CNN's Salma Abdelaziz and Holly Yan and journalists Omar Muqdad and Mohamed Fadel Fahmy contributed to this report.