U.S. warplanes carried out one of the heaviest nights of bombing in Afghanistan since the air assaults began, even as aid groups warned that an increasing number of civilians were being harmed.

The punishing strikes – with 20 bombs dropped in 11 hours – late Friday and early Saturday followed the murder of a key anti-Taliban leader.

Abdul Haq was executed Friday for treason by Taliban soldiers. He was reportedly on a U.S.-backed mission inside Afghanistan to convince tribal leaders to drop their support of the Taliban.

Air base used by Northern Alliance soldiers

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There were reports that U.S. jets, on their 21st day of attacks, hit a Taliban ammunition dump within a military compound, sending fireballs into the sky.

Warplanes were also heard in northern Afghanistan, where Haq was reportedly executed. The jets targeted Taliban troops, who are facing off against the opposition Northern Alliance.

Red Cross warehouse in Kabul

Haq's death came as the Pentagon admitted that its bombs mistakenly hit the same Red Cross warehouse in Kabul twice in the same week.

They blamed "human error" for the mistakes.

Red Cross officials say their warehouses were reduced to rubble, along with food, blankets and supplies for 55,000 people.

They say the buildings were clearly marked with the international Red Cross symbol, and that the bombing is a violation of international humanitarian law.