(CNN) - The Syrian regime has carried out a "systematic" series of abuses against protesters that could "qualify as crimes against humanity," and the United Nations must hold the government accountable, a leading humanitarian watchdog organization said Wednesday.

Human Rights Watch makes these assertions in a report titled "'We've Never Seen Such Horror': Crimes against Humanity in Daraa."

The 57-page document contains chilling detail from dozens of victims and witnesses to abuses in Daraa province, the southwestern Syrian powder keg where the anti-government protest movement began in mid-March before spreading across the country. The government reacted with a tough crackdown against protesters.

At present, the report says, there have been about 887 deaths "across Syria," including at least 418 people in Daraa province.

"For more than two months now, Syrian security forces have been killing and torturing their own people with complete impunity," said Sarah Leah Whitson, Middle East director at Human Rights Watch.

"They need to stop - and if they don't, it is the Security Council's responsibility to make sure that the people responsible face justice."

Human Rights Watch said the Syrian government should take "immediate steps to halt the excessive use of lethal force by security forces" and that the U.N. Security Council "should impose sanctions and press Syria for accountability."

If the government "doesn't respond adequately," the council should refer Syria to the International Criminal Court, the group said.

One major diplomat on Wednesday echoed the international outrage over Syria's crackdown and raised the issue of ICC involvement.