ISLAMABAD, PAKISTAN - Dozens of Afghan forces are said to have been killed in repeated insurgent attacks over the past week in western Farah province, which borders Iran.

Politicians and residents in the province have told local media that Taliban insurgents also have captured several security outposts in and around the Bala Buluk district.

While officials have not yet discussed battlefield details, a provincial police spokesman told Ariana News channel Afghan forces, backed by airpower, have killed more than 30 Taliban assailants and destroyed a number of their military vehicles in ongoing counteroffensives.

Farah also shares a border with Helmand province, Afghanistan's largest, where the Taliban controls or influences a majority of the districts.

The fighting in the western Afghan province comes amid allegations Iran and Russia are providing support to the Taliban.

U.S. Defense Secretary Jim Mattis, while speaking on Thursday during his first visit to Kabul, warned both countries against such activities.

"Those two countries have both suffered losses due to terrorism. So I think it would be extremely unwise to think that they can somehow support terrorists in another country and not have it come back to haunt them," Mattis said, but he declined to discuss specifics.

Tehran and Moscow have acknowledged maintaining contacts with the Taliban to use the group to counter emerging threats from Islamic State in volatile Afghan regions. But both countries deny they are militarily supporting the insurgents.

Stepped up insurgent attacks across most of the country's 34 provinces continue to inflict heavy casualties on Afghan National Defense and Security Forces (ANDSF).

Earlier this week, at least 12 Afghan forces were killed when a Taliban suicide bomber detonated his explosives-packed vehicle in the Maroof district of southern Kandahar province.

The Taliban has also ambushed and assassinated five Afghan district police chiefs in September.

The Islamist insurgency has extended its control or influence to more than 40 percent of the Afghan territory since the U.S.-led foreign combat troops left the country in 2014.