Security forces have been shelling rebel-held towns in Syria, activists say, as President Bashar al-Assad vowed to continue fighting "terrorism".
President Barack Obama said the situation in Syria was "heartbreaking", but that there was no "simple solution".
More than 1,500 Syrians have crossed into Lebanon in recent days, the UN's refugee agency says, amid reports of Syrian troops committing atrocities in Homs.
The BBC's Jim Muir is in Beirut, where he's been following the conflicting coverage of what's happening in Syria.
Muir noted that Baroness Valerie Amos, the special UN envoy for humanitarian affairs has been given permission to visit Damascus Wednesday.
Muir says, "the more crucial mission is the 'make or break one' of Koffi Annan who goes to Damascus on Saturday.
"He's trying to bridge the gap between the two camps into which the world is now polarized over Syria," says Muir. "If he can't do it, I don't believe anyone can and we'll see Syria sliding further into civil war and chaos. "