BEIRUT, Lebanon — Insurgents fired rocket-propelled grenades at the offices of the ruling Baath Party in Damascus on Sunday in a highly symbolic strike that signaled a new chapter in the eight-month uprising against the rule of President Bashar al-Assad.

It was the second attack in days in the capital, which had so far largely been spared the unrest in other cities.

Whether it caused any casualties or significant damage was unclear, but coming amid a series of other assaults, along with growing sectarian strife in Homs, Syria’s third-largest city, it could augur a turn to a protracted armed struggle.

Diplomatic efforts to end the conflict also foundered Sunday as the Arab League rejected a Syrian effort to amend a so-far ineffectual peace agreement, and Mr. Assad said in a newspaper interview that he had no choice but to continue his military campaign against dissidents and vowed that he was prepared to fight and die if needed.