Syrian rebels blew up a tunnel filled with explosives in the northern city of Aleppo on Friday, killing at least 20 pro-government fighters.

The Britain-based Syrian Observatory for Human Rights activist group said the blast took place near the Zahrawi market not far from the citadel in Old Aleppo. A powerful rebel alliance called the Islamic Front claimed responsibility and tweeted a video of the explosion.

Now in its fourth year, Syria's conflict has killed 160,000 people and caused a humanitarian crisis. The United Nations says 9.3 million people need urgent aid inside Syria.

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