BEIRUT, Lebanon — The warring parties in Syria have launched newly assertive attacks on several fronts in recent days, seeking to gain ground and psychological advantage ahead of an intensified United States campaign against extremist Islamic State militants that could include the first American airstrikes inside Syria.

On Tuesday, fighters with the Islamic State, also known as ISIS, said they had shot down a Syrian military aircraft over their stronghold in the northern city of Raqqa, in what antigovernment activists said was a first for the Islamic State.

Syrian-led insurgent groups have brought down numerous Syrian military planes and helicopters in the past, but such attacks have grown rare recently as rivals of the Islamic State struggle to maintain arms supplies. The downed plane crashed into a house, killing eight occupants, said an activist in Raqqa who refused to give his name for fear of reprisal.

Amid a three-year civil war that pits the government of President Bashar al-Assad against many insurgent and often rival factions, new attacks by the government in eastern Syria and in Damascus by opponents of the Islamic State demonstrated the volatility of the situation on the ground. It also showed the difficulty for the United States in striking the militants without allowing the Syrian government or Qaeda-linked insurgents to take advantage of any weakening of the Islamic State.