* Rebel forces lay siege to town dominated by Alawites
* Sunnis felt disenfranchised under Assad, an Alawite
* Shi'ites, Christians are also minorities in Syria
By Louis Charbonneau
UNITED NATIONS, Dec 20 (Reuters) - The U.N. anti-genocide envoy warned on Thursday that minority groups in Syria, including Syrian President Bashar al-Assad's fellow Alawites, are at risk of major reprisal attacks as the 21-month-old conflict escalates and sectarian violence increases.
"I am deeply concerned that entire communities risk paying the price for crimes committed by the Syrian government," Adama Dieng, U.N. special adviser on the prevention of genocide, said in a statement.
The statement said Alawites and other minorities in Syria were increasingly under threat of large-scale reprisal attacks because they are widely associated with the government and allied militia.
Rebels began to push into a strategic town in Syria's central Hama province on Thursday and laid siege to at least one town dominated by Alawites, activists said.
Opposition sources said rebels had won some territory in the strategic southern town of Morek and were surrounding the Alawite town of al-Tleisia.
The rebel operation risks inflaming already raw sectarian tensions as the revolt against four decades of Assad family rule - during which the president's Alawite sect has dominated leadership of the Sunni Muslim majority - rumbles on.
"I urge all parties to the conflict to adhere to international humanitarian and human rights law, which prohibits the targeting of individuals or groups based on religious or ethnic identity as well as attacks against civilians not taking direct part in hostilities," Dieng said.
"I also call on all actors to condemn hate speech that could constitute incitement to violence against communities based on their religious affiliation," said Dieng.
Sunnis have seen themselves as disenfranchised by Assad's Alawite minority, an offshoot of Shi'ite Islam that dominates Syria's power and security structures. Shi'ites and Christians are also sizable minorities in Syria.
U.N. human rights investigators said in Geneva on Thursday that Syria's conflict was becoming more sectarian, with more civilians seeking to arm themselves and foreign fighters - mostly Sunnis - flocking in from 29 countries.
"Reprisal attacks, hate speech and incitement to violence against a particular community have, in the past, been precursors to serious and massive violations of human rights and international humanitarian law," Dieng said.
The U.N. warning comes as al Qaeda's Sunni-dominated affiliate in Syria, al-Nusra Front, which the United States designated a terrorist group last week, has proven itself to be a formidable fighting machine.
He added that the Syrian government was "manifestly failing to protect its populations" and urged the U.N. Security Council, which has been hopelessly deadlocked on the issue since Russia and China have repeatedly used their veto powers to block council action, to overcome its impasse.
Dieng echoed calls by U.N. human rights chief Navi Pillay that the Security Council should refer the case of the Syrian conflict, which has killed over 40,000 people, to the International Criminal Court. (Reporting by Louis Charbonneau; Editing by Lisa Shumaker)