The Islamic State (ISIS) is expanding its reach into Afghanistan and is said to have assumed control over four districts.

ISIS has taken advantage of the country's vulnerability after the withdrawal of British and American troops as well as the fragmentation of the Taliban, the dominant Islamist militant force in Afghanistan, the Express reported.

ISIS was able to capture the areas south of Jalalabad after some 1,600 ISIS fighters launched an assault, according to the Times.

Tens of thousands of Afghan villagers are fleeing their homes as the extremist group "spreads its tentacles and imposes a brutal regime on the country," the reports said.

The jihadis are reportedly conducting public beheadings, extortion and ordering a strict following of Quranic teachings even alien to Afghanistan.

"We are aware of the presence of [ISIS] affiliated militants in Afghanistan, and we are monitoring closely to see whether their emergence will have a meaningful impact on the threat environment in the region,'' a Pentagon spokesman said.

The Pentagon official said Afghan security forces are losing 500 men every month as they battle to stem the tide.

A United Nations report also revealed that the number of groups and individuals who are openly declaring either loyalty to or sympathy with ISIS continues to grow in a number of provinces in Afghanistan.

The report added that ISIS-backed groups "regularly engage" Afghan military forces.

Prominent fighters who once had close ties to the Taliban and al-Qaeda have shifted their loyalties to ISIS in recent months, the U.N. report said.

Taliban leader Mullah Akhtar Mansour was reportedly killed in battle earlier this week, but the Taliban denied the claim.

Shashank Joshi, senior research fellow at the Royal United Services Institute, told the Times that the development gives ISIS a fresh advantage. "Their best chance is to exploit grievances within the Taliban, and that is best done in the environment we see now."

"The Taliban are vulnerable to having entire networks defecting en masse, not out of natural loyalty towards Islamic State, but out of discord within their own organisation,'' he added. "That would be the quickest way for Islamic State to amass strength and it is how they have done so elsewhere."

The U.S.-led coalition has started wide-scale airstrikes campaign in Syria to combat the Islamist group. Britain's RAF jets have begun combat operations over the ISIS heartland, reportedly killing an ISIS sniper team in one mission over Iraq.

Germany, France and Russia have also joined the campaign after suicide bombings in Paris that left 130 people dead.