Ethnic Uzbek warlord Abdul Rashid Dostum's forces swept into Maymana, the center of Faryab province, some 260 miles northwest of Kabul, Interior Minister Ali Ahmad Jalali said.
"Today (Thursday), at 10 o'clock, militia troops loyal to Gen. Dostum entered Maymana city," said Jalali. "They have control of the city."
Jalali said there were no reports of casualties and that the city was calm, but Dostum aides said guards had fired on a crowd, killing four, as Gov. Enayatullah Enayat was rushed to a nearby airport.
Fighters swarming in front of the offices of the Kabul-appointed governor fired into the air and threw rocks at the building, the minister said.
"What Gen. Dostum has done is against all military rules and the constitution of Afghanistan," Jalali told a news conference.
The fighting was bad news for Karzai, and sure to lead to deep concern in Washington over the future stability of Afghanistan, just as American forces are facing an.
The city of Maymana fell before the arrival of hundreds of U.S.-trained Afghan soldiers, who left Kabul for Faryab on Thursday afternoon.
It was the second major burst of militia violence to rock Afghanistan in less than a month, and threw into further doubt this country's readiness for national elections scheduled for September.
In Kabul, trucks and pickups had ferried hundreds of green-bereted Afghan National Army troops armed with assault rifles and machine-guns to the airport.
Some clambered into a transporter provided by the U.S.-led military coalition, which flew them toward Faryab, while others waited for a second aircraft.
Jalali said they would be in position "soon," but gave no details of what action they would take. "The arrival of the ANA will improve the situation" and pave the way for a full investigation, he said.
National police would also be sent to Maymana, he said.
Dostum ran a large chunk of the country as a personal fiefdom during much of Afghanistan's brutal civil wars.
He returned to power in the region after helping U.S. forces drive out the Taliban in late 2001, and has maintained a large private army ever since.
But he has appealed in vain for a top security job in Karzai's administration and his men have fought repeatedly with Tajik rivals allied with Defense Minister Mohammed Fahim for control of territory.
Officials in Faryab accused Dostum of trying to drive them out of office for allying too closely with Karzai's government.
A Dostum aide in Kabul said he had discussed the situation in Faryab with elders from that province, but ordered no moves against Enayat or Hashim Khan, the commander of the 200th Afghan army division, who Jalali said had also fled Maymana.
The aide, Akbar Boy, said government troops were welcome in the region, but suggested there would be a violent backlash if they sided with the embattled officials, who he accused of using government funds to try to buy votes and influence ahead of September's elections.
"The people of Faryab will rise against them," Boy said. "They don't want Hashim."
The clash comes less than a month after the government sent 1,500 soldiers to the western city of Herat after bloody battles between rival factions left 16 people dead, including a Cabinet minister.
The spreading violence, combined with a stubborn Taliban-led insurgency in the south and east, has compounded concern that September's elections will be spoiled by militant attacks and voter intimidation.
The government has vowed to disarm some 40,000 militia fighters and round up heavy weapons around the country in time for the vote.
"The only way to solve these problems is DDR," Jalali said, referring to a U.N.-sponsored demobilization scheme to disarm fighters and reintegrate them into civilian life. "We hope that by the election, we will have everything under control."
Meanwhile, clashes in Gereshk, some 220 miles southwest of the capital Kabul, in Helmand province left at least seven people dead, including four Afghan soldiers and police officers in the country's insurgency-torn south, officials said Thursday.
A militant and one Afghan soldier were killed in a gunbattle that also left an American soldier and a second Afghan wounded, the U.S. military said. The soldiers were not identified and no details of their injuries were given.
The shooting broke out Wednesday during a joint Afghan-U.S. operation near, military spokeswoman Michele DeWerth said.
"U.S. troops were conducting a cordon and search when they came under attack," DeWerth said. "Four anti-coalition militia (were) detained and one killed."
DeWerth said the dead Afghan soldier was a member of the new U.S.-trained Afghan National Army.