At least 20 people were killed and 21 others were injured when a passenger bus skidded off a mountainous terrain and plunged into a ravine in northwest Pakistan on Friday, authorities said.The incident occurred on the Karakoram Highway in Diamer district of Gilgit-Baltistan region when the bus was on its way from Rawalpindi to Hunza. The driver lost control of the vehicle which then overturned and landed near the banks of the Indus River, a police official said.As many as 43 passengers were aboard the bus when the incident took place, the official said, adding that the death toll was expected to rise further as many of the injured were in critical condition.