BEIRUT — Warplanes unleashed airstrikes on the suburbs of the Syrian capital and near the northern, rebel-held city of Aleppo on Friday, hours before a cease-fire brokered by the United States and Russia was to go into effect at midnight local time across the war-ravaged country.

The barrage came as the main Syrian opposition and rebel umbrella group said dozens of factions — 97 groups in all — have agreed to abide by the cease-fire. The High Negotiations Committee, or HNC, said a military committee has been formed to follow up on the cease-fire.

The Britain-based Syrian Observatory for Human Rights said the warplanes in Friday’s strikes were believed to be Russian. The Kremlin did not comment the latest developments but denied allegations that the Russian air force bombed civilian positions east of Damascus the previous day.

The rebel-held Damascus suburb of Douma was hit 40 times on Friday.