Forces of ethnic Uzbek strongman General Abd al-Rashid Dustum invaded Faryab province on Wednesday, prompting the central government to dispatch national army troops there on Thursday in an attempt to restore order.
"Both the governor and the commander have fled. Dustum's forces have overrun Maimana," said a defence ministry official, who did not want to be identified.
Presidential spokesman Jawed Ludin said an emergency National Security Council meeting chaired by Karzai had heard the city fell "to irresponsible armed individuals from neighbouring provinces and areas around Maimana".
United Nations spokesman Manoel de Almeida e Silva said UN staff in Maimana had reported fighting in the city on Thursday, but he had no further details.
The Pakistan-based Afghan Islamic Press news agency said a batch of national army troops had reached Mainama airport, but had not ventured into the city.
A soldier from the force of General Muhammad Hashim Habibi, the commander of the provincial militia who fled with Governor Anayat Allah Anayat, said Habibi was currently in Belcheragh, a town about 40km east of Maimana.
"We would like to ensure security for the people and uphold the authority of the central government"
The government has defended Anayat as its legitimate representative in Faryab, and said if an investigation determined Dustum had acted against him, it would be deemed "an unlawful act".
Spokesman Ludin said earlier the deployment of the national army would be "just one measure" to ensure the situation did not worsen.
"General Dustum is an adviser to the President. However, that does not give him the right to deploy forces or get involved in any military operational issues," he said.
"We would like to ensure security for the people and uphold the authority of the central government," he said. "The people need to know they will not be left to the mercy of warlords."
Dustum's forces have been involved in several rounds of fighting for territory since the overthrow of the Taliban in late 2001. A federalist, he unsuccessfully opposed Karzai's attempt for a strong presidency in a constitutional assembly in January.