"By the grace of Allah the Almighty, the soldiers of the Caliphate attacked a border guard post of the Tajik apostates in the Ishkobod area, near the Tajik-Uzbek border," the IS group said in a statement released online.
Tajik authorities on Wednesday said that fifteen jihadists were killed during an attack on a border post that officials blamed on members of the Islamic State group who crossed over from Afghanistan.
The overnight assault also left a soldier and policeman dead, authorities in the ex-Soviet republic said.
The Islamic State group claimed that its fighters killed 10 soldiers in the clashes.
In a separate statement, Amaq, the IS propaganda agency, said that all the IS attackers were killed but did not specify how many or where they had come from
However "the whole story is a bit murky,", Raffaello Pantucci, senior associate fellow at the London-based Royal United Services Institute, told AFP.
"Everyone I have asked in Tajikistan is suspicious."
- 'There are dozens of questions' -
Tajikistan's interior ministry insisted Friday that the attack was perpetrated by IS fighters -- "mostly citizens of Tajikistan" -- who had crossed into the country from Afghanistan.
Pantucci said that the IS claim was a boost to the group after leader Abu Bakr al-Baghdadi was killed in a US raid.
"For ISIS it looks good to claim," he said.
"A new leader comes in and we immediately see attacks."
IS also released Friday what it said was a video of the attackers reciting a pledge of allegiance to the group's new leader Abu Ibrahim al-Hashimi al-Quraishi.
Before the Islamic State 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.
Tajik authorities 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.
"All of them are members of the Islamic State," the border guard service said.
But some also refused to take the Tajik authorities' statements at face value.
Daniil Kislov, director of the Moscow-based Fergana News agency which covers Central Asia and Afghanistan, also poured cold water on Tajikistan's claims.
"There are dozens of questions," Kislov told AFP before the IS group 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," he said.
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 Islamist groups in Syria and Iraq.
Tajikistan's ally Russia, which maintains a military base in the country, has warned over potential IS incursions from Afghanistan into the region.
On Thursday, Alexander Bortnikov, head of Russia's FSB security service, said IS was setting up a base in Afghanistan to target ex-Soviet countries.
"We are seeing increased activities of IS branches in Afghanistan," Bortnikov said.
"Their goal is to increase a base to expand into the Commonwealth of Independent States," Bortnikov said, referring to a grouping of ex-Soviet states.
Last summer IS jihadists who were residents of Tajikistan claimed a hit-and-run that killed four Western tourists on a cycling trip.
Edward Lemon, a Central Asia expert at the Daniel Morgan Graduate School in Washington, also said any claims by Tajikistan in relation to the group should be treated with caution, given the government's track record on extremism.
"Tajikistan has frequently used the threat of extremism and terrorism to undermine its opponents and strengthen its grip on power," Lemon said before the IS claim.