Fierce clashes reported in Syrian capital But the official Syrian Arab News Agency issued a denial, saying reports of fighting in Damascus were "categorically baseless" and "desperate attempts intended to lift the morale of the terrorists." Also Wednesday, two suicide car bombs struck offices of Syrian military intelligence and the Interior Ministry's intelligence arm, the General Security Directorate, in the central city of Palmyra, killing at least 12 people and injuring more than 20, according to the Britain-based Syrian Observatory for Human Rights, a watchdog group that monitors the violence in the country. State media confirmed the attacks but said the car bombs targeted a residential area in the city. A video posted online shows a black plume of smoke over what appears to be a residential district shortly after one of the explosions. The stepped-up violence Wednesday came two days after Mouaz al-Khatib, head of the Syrian Opposition Coalition, called again on the Assad government to begin negotiating to end the civil war. Many prominent opposition activists have rejected any talks with the government before Assad steps down. Khatib, in an interview Monday with the al-Arabiya satellite news channel, said he is willing to meet with Vice President Farouk al-Shara. But last week he laid out tougher terms in a Facebook posting, asking the Syrian government to release about 160,000 political prisoners and to renew the passports of activists outside the country. While the government has not officially responded to Khatib's request, a prominent lawmaker, Fayez Sayegh, told the Associated Press on Tuesday that any talks must be conducted without preconditions. Khatib's offer of dialogue has not been entirely popular within the opposition. Some members of the Syrian Opposition Coalition initially dismissed the idea of talks last week, and the Syrian Islamic Front, an umbrella group of Islamist fighters, issued a statement Wednesday criticizing Khatib and rejecting any negotiations with the Assad government. "Mouaz al-Khatib started an initiative that will lead to our people losing their rights and destroy many of the gains that the revolution accomplished in recent days," the statement said. The Syrian Islamic Front is one of several Islamist factions that fought in a key battle to take over the Taftanaz military air base in northwestern Syria in mid-January. Another umbrella group of Islamist factions, Liwa al-Islam, said it took part in the assault in Damascus on Wednesday. A statement on the group's Facebook page called the operation "the battle to enter Damascus." Islam Alloush, a spokesman for the group, said fighters in the assault were using antiaircraft guns and other heavy weaponry confiscated from the Syrian military. A reporter with the pro-opposition Shaam News Network who uses the pseudonym Abu Hassan al-Demeshqi said fighters from the rebel Free Syrian Army, who also took part in the assault, used confiscated tanks to attack government checkpoints. "We witnessed extremely heavy clashes in the streets," he said. By late afternoon Wednesday, rebel fighters had taken over a handful of checkpoints in eastern and southern Damascus, according to opposition activists. Government forces retaliated with artillery and mortar fire in the neighborhoods where the most intense clashes took place, including Jobar, Zamalka and Tadamon, according to opposition activists. One video posted online shows a fighter blasting a heavy machine gun from the back of a pickup truck. Other videos show burning residential buildings in the capital, as well as a mosque in the Jobar neighborhood streaming thick, black smoke from its minaret. The Local Coordination Committees activist network said at least 55 people were killed in Damascus and its suburbs Wednesday. Damascus residents interviewed by telephone described a ramped-up security presence across the city, with many checkpoints set up to search cars and some roads blocked entirely. One young woman working in central Damascus, who spoke on the condition of anonymity out of fear for her safety, said her office was closed and the staff was sent home after clashes broke out nearby. "It looks like Judgment Day is upon us," said a resident of a western Damascus neighborhood who also asked not to be named for security reasons. The sounds are extremely loud and heavy, and it didn't stop since this morning, not for a single second. Ahmed Ramadan and Suzan Haidamous contributed to this report.