Explosions and gunfire rocked the Syrian capital early Thursday, an opposition group said, a day after military defectors struck a government intelligence complex in a bold assault reflecting surging resolve. No other details were immediately available on the explosions Thursday. The reported blasts come as international leaders intensify their pressure on President Bashar al-Assad to end violence against protesters in the uprising the United Nations says has killed 3,500 people. The defector group Free Syrian Army said it attacked an air intelligence base in Harasta and planted "powerful explosions inside and around the compound that shook its foundations." Air intelligence has been deeply involved in the eight-month crackdown by the Syrian government against protesters, said Andrew Tabler, an expert at the Washington Institute for Near East Policy. The strike reflects the growing sophistication of the Free Syrian Army, according to Tabler. "It opens up a new era of the conflict," he said, adding that the development represents "a bad direction" for the country. "Until now, most of the protests have been peaceful." The defector group said it attacked several areas in Damascus to foil future government attacks against civilians. Activists said the deserter army used rocket-propelled grenades to damage the intelligence complex in the eastern suburb of the capital, Damascus. Opposition groups have urged the United Nations to impose a no-fly zone, as it did in the Libyan conflict, to help protect protesters. Army leaders have said the measure would allow them to establish a base of operations to launch a campaign to bring down al-Assad's regime. The Arab League, meeting in Rabat, Morocco, said al-Assad didn't stick by his pledge to withdraw armed forces from populated areas and allow journalists and monitors unfettered access. A senior league official said the group gave Damascus three days to implement a protocol to allow observes to enter the nation and verify whether Syria has taken measures to protect civilians. The league has also called on member states to withdraw their ambassadors from Damascus, a decision that will be up to each nation. In a sign of growing international isolation, France withdrew its ambassador Wednesday after attacks on its missions in the nation. As the standoff rages on, at least 22 civilians were killed Wednesday, according to the Local Coordination Committee of Syria, a coalition of activists. CNN is not able to independently verify claims of fighting and casualties because the Syrian government has restricted international media access to the country.