Twin suicide bombings killed at least nine people in Syria on Monday, fueling fears that Islamist extremists are trying to muscle in on the country's 13-month-old pro-democracy uprising. United Nations chief Ban Ki-moon condemned the "terrorist bomb attacks," which are part of a new wave of violence threatening to undermine a U.N.-brokered ceasefire between the government of President Bashar al-Assad and opposition groups. Are jihadists hijacking the revolt? Here, a brief guide:

How common are suicide bombings in Syria?
They were unheard of until recently, but there have been at least eight since December, including the two attacks on Monday and another last Friday that killed at least 10 people. The twin bombings on Monday struck near a security compound in northern Syria. On the same day, rockets hit the central bank in Damascus — another indication that attacks are getting more sophisticated and more violent.

And terrorists are to blame?
Maybe. U.S. intelligence officials say the shift in tactics could indicate that al Qaeda leaders are trying to seize a role for themselves and co-opt what started out as a peaceful uprising. "Al Qaeda was caught somewhat flatfooted in the Arab Spring," one official tells The Wall Street Journal. They may not let the same thing happen in Syria.

But who exactly is responsible for these attacks?
It's hard to say with certainty, although a Salafi Muslim jihadist group called Jabhat al-Nusra has claimed responsibility for several (though not all) of the recent suicide bombings, including the one in Damascus last week and twin attacks that killed 27 people in Damascus on March 17. 

Does this mean jihadists have joined the opposition?
Not exactly. Some rebels concede that they're resorting to homemade bombs, which have proven effective in Iraq and Afghanistan, to give their outgunned forces a fighting chance against Assad's massive army. Other opposition leaders charge that the government is staging the suicide attacks to support its claim that the military is cracking down on "al Qaeda terrorists" and "armed terrorist gangs," not peaceful protesters and innocent civilians. "The only al Qaeda cells that operate in Syria," says U.S.-based opposition activist Ammar Abdulhamid, "are those manipulated by Assad's security apparatuses."

Sources: Associated Press, Christian Science Monitor, Reuters(2), Wall Street Journal