BAGHDAD — An obscure Kurdish militant group that is an offshoot of the Kurdistan Workers’ Party, which has long waged an insurgency inside Turkey, claimed responsibility on Friday for a car bombing this week in Ankara that killed 28 people. The group said the attack was in revenge for the Turkish Army’s campaign against Kurdish insurgents in the country’s southeast.

The group, known as the Kurdistan Freedom Hawks, made its claim as Turkish leaders continued to lay blame for the attack on a different group: a Syrian Kurdish militia that is supported by the United States in the battle against the Islamic State.

Turkey and the United States, two NATO allies, have cooperated in some ways against the Islamic State, but they have diverged over the role of the Kurds. The United States has seen them as a reliable ally within Syria in combating the Islamic State, while the Turks have seen them as a national security threat, complicating American efforts in that campaign.

In blaming the Syrian Kurdish group, known as the People’s Protection Units, for the bombing in Ankara, the capital, Turkey was putting pressure on the United States to cut off support for the group, an outcome that analysts and American officials have said is unlikely given the group’s success against the Islamic State, also known as ISIS or ISIL.