"An intense battle is going on in the city", he said without providing additional details.
"You've got Army, police, commandos and Afghan air force involved in the fight", O'Donnell said. "They are roaming in the intersections".
The NATO RS mission was established in 2015 with non-combat mission to train, advise and assist the Afghan National Defense and Security Forces (ANDSF), who assumed nationwide responsibility for Afghanistan's security following the conclusion of the previous NATO-led International Security Assistance Force (ISAF) mission. "The worldwide coalition, led by the United States, is focused on providing the military pressure, in conjunction with social pressure and diplomatic pressure that will force them to come to the table", Votel said during a visit to Tajikistan's capital, Dushanbe.
Farah is a poppy-growing province in an isolated region of Afghanistan. In recent months, the US has deployed additional advisers and carried out thousands of airstrikes in a bid to bolster the security situation in a strategy of renewed aggression.
Unconfirmed reports said the Taliban had seized Farah's central prison and freed several hundred inmates, including dozens of militants.
Gen. Mohammed Radmanish, chief spokesman for the Defense Ministry, said the situation "was under control".
Clashes have been ongoing for the past few months in different parts of Farah province.
Mohammad Sarwar Osmani, a lawmaker from Farah province, said strategic security checkpoints around the city were overrun by Taliban and the insurgents were near police headquarters and the governor's offices.
He said their targets are only the military facilities.
The assault comes as the Taliban are stepping up their spring offensive, in an apparent rejection of a peace talks overture by the Afghan government.
A senior Afghan police official, reached by telephone inside Farah city, described the government's situation as "out of control" and predicted the insurgents would renew their offensive under cover of dark, when air support was less effective against them.
A doctor from the provincial hospital in Farah, who did not want to be identified, said so far two dead and 16 wounded members of the Afghan security forces, as well as five wounded civilians including women and children, had been taken to the hospital.
"The Afghan security forces are bringing their full capabilities, consisting of Army, Police, Commandos and Air Force, to bear on the situation", it said, adding that US forces "stand firm beside our Afghan partners and will continue to reinforce this Afghan-led offensive".