BEIRUT, Lebanon (AP) — Syrian rebels launched an assault at dawn on Saturday on a strategic air base in the north of the country, trying to disrupt strikes by warplanes and helicopters that pound rebel-held towns and give the government of President Bashar al-Assad a major edge in the 20-month-old civil war.

The assault, reported by activists, came a day before the start of a key international conference in Qatar at which the United States and its allies aim to reorganize the opposition’s political leadership and unite its ranks. The leadership in exile has been widely seen as ineffective and out of touch with rebel fighters on the ground.

Rebel forces attacked the Taftanaz air base early on Saturday morning in fighting with government forces that continued into the afternoon, said the Britain-based Syrian Observatory for Human Rights, an opposition group.

Joining Syrian rebels in the attack were fighters from Jabhet al-Nusra, an Islamic militant group inspired by Al Qaeda and made up of foreign jihadis, according to the observatory. Nusra fighters, who are considered among the most experienced and disciplined among the opposition forces, have led attacks on other air bases in the north in past months.