Thirty-four Tajik troops have been killed in two separate incidents during a military operation against Islamic insurgents in the east of the Central Asian state, military sources said Thursday.

The heavy toll in a single day in what official statements insisted were accidents is a severe blow for the government as it seeks to quell mounting Islamist unrest in the remote Rasht Valley in the east of the country.

Twenty-eight troops were killed Wednesday when a military helicopter crashed in a possible militant strike, military sources told AFP, although public statements insisted only four were killed in an accident.

"Not a single passenger survived the crash of the Mi-8 military helicopter on Wednesday morning," said a military source, speaking on condition of anonymity.

"Twenty-eight members of the elite Alfa anti-terrorism division and officers of the Tajik national guard were killed," the source said, raising an earlier toll of 25.

A high-ranking Tajik military source, also speaking on condition of strict anonymity, said that it was possible that the helicopter had been shot down by a missile fired by Islamists from their mountain hideouts.

"It is possible that the Islamists could have shot down the Mi-8 from the mountains when it was not far from landing at Rasht airstrip," the source said.

But the official statement said the helicopter had hit a power line, sending it crashing into a river. The helicopter was carrying the servicemen from the capital Dushanbe to the Rasht Valley to take part in the military operation.

Meanwhile, another six troops were killed in a separate incident caused by the accidental explosion of a mine late Wednesday, according to a military source.

"The faulty placement of a mine led to an explosion that killed six Tajik national guards," the source told AFP, adding that more than 10 officers were wounded in the incident in the Rasht Valley.

The spike in unrest in Tajikistan, which shares a porous 1,300-kilometre (800-mile) border with Afghanistan, has followed the escape of 25 Al-Qaeda-linked militants from a prison in a brazen nighttime getaway in August.

Twenty-eight soldiers were then killed in an ambush on a military convoy in the Rasht Valley in September. The Islamic Movement of Uzbekistan (IMU), an Al-Qaeda-linked militant group, later claimed responsibility for the attack.

After the militants failed to respond to a government ultimatum, Tajikistan launched a huge military operation to hunt down the Islamists behind the ambush, with officials saying that a dozen militants have already been killed.

Tajikistan is a majority-Muslim country and the poorest state to emerge from the collapse of the Soviet Union nearly two decades ago.

It was hit by a devastating civil war between Islamists and backers of Tajik President Emomali Rakhmon that broke out after the collapse of the Soviet Union and only ended in 1997 with the loss of tens of thousands of lives.

The IMU -- branded a terrorist organisation by the United States -- was founded in the late 1990s in Tajikistan with the goal of overthrowing Uzbek President Islam Karimov and creating an Islamic Sharia law state in the ex-Soviet republic.