BAGHDAD — The U.N. refugee agency said Friday it was rushing aid to thousands of Christians who fled a northern Iraqi city, while a prominent Shiite cleric appealed for unity as lawmakers consider a U.S.-Iraq security deal.

Some 13,000 Christians have been chased away by threats and extremist attacks in Mosul this month, said Ron Redmond, spokesman for the U.N. High Commissioner for Refugees.

That number is over half the community in a city where Christians have lived since the early days of the religion.

"Many left with little money and need help," Redmond told reporters in Geneva, where the agency is headquartered.

He recounted the story of a Christian nurse who told his agency the threats started months ago with phone calls, letters and messages left on doors.

Another woman said she fled when she heard of a Christian who was murdered.

"We were the hard core that never wanted to leave Iraq, even with the tense environment," the woman, who fled to Syria, was quoted by the agency as saying.

The agency has delivered relief supplies to more than 1,700 Christian families now displaced in the north of the country, Redmond said. Most are living in churches, monasteries and the homes of relatives in nearby Christian villages and towns.

No group has claimed responsibility, but Sunni extremists are believed to be behind the campaign, which is taking place despite U.S.-Iraqi operations aimed at routing insurgents from remaining strongholds north of the capital.

Attacks against Christians and other minorities had tapered off amid a drastic decline in overall violence nationwide, before picking up again this month.

In Baghdad, meanwhile, a hardline Shiite cleric and lawmaker called for rational debate on a draft U.S.-Iraqi security pact that would give the United States a legal basis for keeping its forces in Iraq for three more years.