MOSUL, Iraq -- The letter tossed into Mustafa Abu Bakr Muhammad's front yard got right to the point.
"You will be killed," it read, for collaborating with the Kurdish militias. Then came the bullet through a window at night.
A cousin had already been gunned down. So Muhammad and three generations of his family joined tens of thousands of other Kurds who have fled growing ethnic violence by Sunni Arab insurgents here and moved east, to the safety of Iraqi Kurdistan.
"We had our home in Mosul and it was good there, but things are now very bad between Arabs and Kurds," said Muhammad, 70, standing outside his new, scorpion-infested cinderblock house in the nearby town of Khabat.
While the US military is trying to tamp down the vicious fighting between rival Arab sects in Baghdad, conflict between Arabs and Kurds is intensifying here, adding another dimension to Iraq's civil war. Sunni Arab militants, reinforced by insurgents fleeing the new security plan in Baghdad, are trying to rid Mosul of its Kurdish population through violence and intimidation, Kurdish officials said.
Mosul, Iraq's third-largest city, with a population of 1.8 million, straddles the Tigris River on a grassy, windswept plain in the country's north. It was recently estimated to be about a quarter Kurdish, but Sunni Arabs have already driven out at least 70,000 Kurds and virtually erased the Kurdish presence from the city's western half, said Khasro Goran, the deputy governor of surrounding Nineveh Province and a Kurd.
The militants "view this as a Sunni-dominated town, and they view the Kurds as encroaching on Mosul," said Colonel Stephen Twitty, commander of the 4th Brigade, 1st Cavalry Division, which is deployed in Nineveh.
Some Kurdish and Christian enclaves remain on the east side, though their numbers are dwindling. Kurdish officials say the flight has accelerated in recent months, contributing to the wider ethnic and religious partitioning that is taking place all over Iraq.
Nineveh is Iraq's most diverse province, with a dizzying array of ethnic and religious groups woven into an area about the size of Maryland. For centuries, Arabs, Kurds, Christians, Turkmens, Yezidis, and Shabaks lived side by side in these verdant hills, going to the same schools, bartering in the same markets, even marrying one another on occasion.
But what took generations to build is starting to unravel in the shadow of the Sunni Arab insurgency, which is tapping into several wells of ethnic resentment.
Already embittered at the toppling of Saddam Hussein's Sunni Arab government, insurgents here have been further enraged by their current political disenfranchisement, the result of their boycotting the 2005 elections. The main Kurdish coalition now holds 31 of 41 seats on the provincial council and all the top executive positions, even though Kurds make up only 35 percent of the province.
Sunni Arabs have asked for new provincial elections and are growing frustrated that the Shi'ite- and Kurdish-dominated national government seems to be ignoring their requests.
Just as worrisome for the Arabs is a growing push by the autonomous region of Iraqi Kurdistan to annex large swaths of eastern and northern Nineveh. A contentious measure in the Constitution gives the regional Kurdish government the right to take the land by the end of 2007 through a popular referendum.
The parts of the province that Iraqi Kurdistan wants are called the "disputed territories" along its border, areas that were historically Kurdish until Saddam Hussein moved in Arabs and forced out half a million Kurds to strengthen Arab control, Kurdish officials say.