The United States has disclosed a secret military ground operation in Syria that killed an Isis operative seen as a close associate of leader Abu Bakr al-Baghdadi and linked to an attack on a Turkish nightclub that left 39 people dead.

Abdurakhmon Uzbeki, who was believed to be from Uzbekistan, was killed during the ground assault near Mayadin on 6 April, said Colonel John Thomas, a spokesman for the US military's Central Command.

“It was a ground operation. I think that's all we're willing to say about that,” Cnl Thomas said, declining further comment. He specified that Uzbeki did not die in an airstrike.

Cnl Thomas said Uzbeki helped facilitate the attack on the exclusive Istanbul nightclub Reina in Istanbul on New Year's Eve. Isis claimed credit for the attack, saying it was revenge for Turkish military involvement in Syria.

The attacker opened fire with an automatic rifle, throwing stun grenades to allow himself to reload and shooting the wounded on the ground. Among those killed in the attack were Turks and visitors from several Arab nations, India and Canada.

“We have clearly linked him with the New Year's Eve Istanbul bombing,” Cnl Thomas said.