Christians are being threatened into supporting the opposition and churches are being desecrated in civil war-ravaged Syria, a US congressional hearing has heard.

Congressman Chris Smith warned that religious minorities in Syria were being killed or persecuted at an alarming rate.

Smith, chairman of the House congressional panel overseeing global human rights issues, said peaceful coexistence among Syria's religious communities had disappeared after the start of the war.

"Before the war, Syria was a fairly pluralistic society, with Alawites, Shias, Ismalis, Yezidis, Druze, Christians, Jews, and Sunnis living in relative peace, side by side," he said.

"The situation was far from perfect, as President Bashar al-Assad's regime had a vast security apparatus in place with members inside each of the religious communities to monitor their activities.

"The Assad government was guilty of serious human rights violations, including the summary imprisonment and execution of political opponents. But relations between the various religious groups were generally not violent. That civil co-existence has ended with the war."

Thomas Melia, Deputy Assistant Secretary of State in the Bureau of Democracy, Human Rights and Labor, told Congress at the June 25 hearing that many Christians were reporting threats on their lives if they did not join the opposition.

Christians have been "driven from their homes and killed en masse as presumed supporters of the regime", he said.

Melia said there was "increasing lawlessness" in northern areas, with regular kidnappings, rapes, and looting. He noted the April 22 kidnapping of Syrian Orthodox Archbishop Yohanna Ibrahim and Greek Orthodox Archbishop Paul Yazigi, who remain missing.

Dr John Eibner, chief executive of Christian Solidarity International, said the deaths of Christians did not appear to be collateral damage but the result of deliberate attacks.

"Victims recounted to me details of the religious cleansing of Christian neighbourhoods in Homs and Qasair by armed jihadis who threatened them with death and the destruction of their property if they did not leave their home," Eibner said.

"Churches in Homs and Qasair have not only been damaged as a result of the exchange of mortars by the Syrian army and rebel forces, but have also been desecrated after falling under the control of the armed opposition."

Watch Congressman Smith's opening remarks at the hearing: