ISIS on Friday claimed responsibility for a deadly attack against a Tajik border post, not far from the capital Dushanbe, earlier this week.
The soldiers attacked a border guard post of the Tajiks in the Ishkobod area, near the Tajik-Uzbek border, the extremist group said in an online statement.
Tajik authorities said on Wednesday that fifteen extremists were killed during an attack on a border post that officials blamed on ISIS members who crossed over from Afghanistan.
The overnight assault also left a soldier and policeman dead, authorities in the ex-Soviet republic said.
The extremist group claimed that its fighters had killed 10 soldiers in the clashes.
In a separate statement, Amaq, the ISIS propaganda agency, said that all the attackers were killed but did not specify how many or where they had come from.
Tajikistan's interior ministry insisted on Friday that the attack was perpetrated by ISIS fighters - "mostly citizens of Tajikistan" - who had crossed into the country from Afghanistan.
Before the group claimed the attack a spokesman for Afghanistan's defence ministry said that it did not believe the event which happened near Tajikistan's border with Uzbekistan had anything to do with Afghanistan.
Authorities in Tajikistan also said five attackers had been captured.
Citing the confession of a detained attacker, the Tajik border guard service said the group crossed from Afghanistan in darkness on Sunday into the Tajik district of Qabodiyon.
The border guard service said all of the attackers were ISIS fighters.
But the Tajik authorities' claims and official photos from the scene were also met with some scepticism from Tajikistan experts.
"There are dozens of questions," Daniil Kislov, director of the Moscow-based Fergana News agency which covers Central Asia and Afghanistan, told AFP before ISIS claimed the attack.
"Where did the attackers find cars, which had Tajik number plates?
"If they stole them, who were the victims (of the theft)? You can go on and on," said Mr Kislov.
Tajikistan, a poor mountainous country of nine million people bordering Afghanistan and China, has been hit by conflicts since it gained independence after the collapse of the Soviet Union in 1991.
Tajikistan and other ex-Soviet Central Asian countries have been major sources of recruits for radical groups in Syria and Iraq.