Mr Hague said Mr Assad's forces had killed civilians mercilessly in a "desperate attempt to cling to power" and blamed a "lack of political will" at both national and international level for failure to tackle the problem.
The Foreign Secretary highlighted efforts to end sexual violence in trouble-spots around the world, and the report also raised concerns about increased levels of discrimination faced by gay people in a number of countries.
Mr Hague, along with Hollywood star and UN special envoy Angelina Jolie, is to host an international summit in London on ending sexual violence in conflict zones later this year.
On Syria, he said: "In some countries a desire for power at any cost fuelled terrible human rights abuses, while a lack of political will at the national and international level meant that not enough was done to prevent them.
"The consequences were most catastrophic in Syria. The Assad regime bears the primary responsibility for protecting its own population. But it continued last year to kill civilians mercilessly with chemical weapons, barrel bombs and artillery fire."