The Syrian Observatory for Human Rights said that of the 20,000 killed, some 6,500 were civilians. It said nearly half of the civilians were killed in strikes by government or Russian forces. The Observatory said that more 33,400 people, including 10,000 civilians, were killed in 2017.
The past year, however, also saw the largest single wave of displacement since the conflict began, with more than a million people driven from their homes in just six months, according to the U.N. By the end of the year the rebels and Islamic militants were largely confined to the northern Idlib province, where a Russian, Turkish and Iranian agreement has for now frozen the battle lines.
A new battle is brewing in oil-rich eastern Syria after President Donald Trump's decision to withdraw some 2,000 U.S. forces who are supporting Kurdish fighters battling the Islamic State group. The Kurdish fighters control nearly 30 percent of Syria.
IS has lost almost all the territory it once held but remains a potent force. Turkey and allied Syrian fighters are meanwhile threatening to launch an offensive against the Kurdish forces, who Turkey views as terrorists because of their links to an insurgent group inside Turkey.
The Syrian conflict began in 2011 as a peaceful uprising against Assad but soon escalated into a devastating civil war that has drawn in foreign powers. The conflict is believed to have killed more than 500,000 people.