KANDAHAR, Afghanistan (Reuters) - Afghan forces backed by air strikes are preparing a new offensive to clear Taliban militants encircling the capital of Afghanistan’s province of Uruzgan, where the two sides have fought bitter clashes, officials said on Saturday.

Street-to-street fighting in Tarin Kot, the provincial capital, where Taliban fighters rampaged to within a few meters (yards) of the government center on Thursday, has quieted as the frontline has moved 15 to 20 km (9 to 12 miles) away, said Dost Mohammed Nayab, the provincial governor’s spokesman.

“We have enough forces, ammunition and reinforcements at the moment,” he said. “We are working on a bigger plan to eliminate the Taliban from Uruzgan. We will have a massive operation in three or four days.”

The Taliban attack, in one of Afghanistan’s top opium-producing areas, reveals how thinly stretched Afghan security forces have become as they try to contain Islamist insurgents fighting to overthrow the Western-backed government in Kabul.

Since the U.S.-led military coalition declared the end of its combat mission in 2014, the Taliban have sought to move from their strongholds in rural areas and attack major cities such as south-central Tarin Kot, as well as the capitals of the southern province of Helmand and the northern region of Kunduz.

On Friday U.S. warplanes conducted at least three air strikes in Uruzgan, alongside strikes by the Afghan air force.

Taliban forces briefly seized Kunduz city a year ago and the American commander of the roughly 16,000 international troops remaining in Afghanistan has vowed to help Afghan forces hold onto the cities.

In an online statement on Friday, the Taliban said clashes continued and at least one local government commander had defected, along with some of his men and weapons.

The group also claimed to have hit a commando helicopter, killing or wounding all aboard. The Afghan military denied that report.