(CNN) -- The militant group that has been ruthlessly fighting to take control of Iraq has apparently posted chilling photos on jihadi Internet forums seeming to show the executions of Iraqi security forces.
CNN cannot independently confirm the authenticity of the images purportedly posted by ISIS, or the Islamic State in Iraq and Syria. CNN is examining the terrain in the images, some of the signage on buildings in several of the pictures and the uniforms of the apparent victims. Those details suggest the photos are real and were taken in Iraq.
A caption on some of the images: "apostates heading to their hole of doom."
ISIS, an al Qaeda splinter group, wants to establish a caliphate, or Islamic state, in the region that would stretch from Iraq into northern Syria. The group has had substantial success in Syria battling Syrian President Bashar al-Assad's security forces.
ISIS's quick advance in Iraq has been helped by many Sunnis who feel that the Shiite-dominated government has marginalized them.
On Friday, a tweet on what was claimed to be an ISIS Twitter account claimed that its members killed at least 1,700 Shiites. CNN was unable to verify the authenticity of the account, and the account appeared to have been taken down Sunday.
The militant group continued its assault Sunday. Iraqi security forces and gunmen believed to be ISIS clashed near al-Khalis, which is about 18 miles from Baquba, police officials in that Iraqi city told CNN.
Baquba is about 37 miles northeast of Baghdad, the capital. ISIS seized Iraq's second-largest city, Mosul, last week and have threatened to march on Baghdad.
Also Sunday, ISIS gained control of two villages in Adhaim, a move that constituted the militants' first push into Diyala province, which is northeast of Baghdad.
Baquba, Diyala's capital, is an ethnically-mixed city, though Sunnis represent a slight majority.
Given Baquba's proximity to Baghdad, if Baquba were to fall, it could give ISIS militants three-pronged access to Baghdad -- from Anbar province to the west, Ninevah and Salahuddin provinces to the north and from Diyala province from the northeast.
In other violence Sunday, three mortar rounds landed in al-Khalis, near a recruitment center opened recently for volunteers to join the Iraqi army in their fight against ISIS. Officials could not provide more details.
And, the northwest Iraqi city of Tal Afar fell to ISIS militants, according to Iraqi Gen. Mohammed al-Quraishi. Many of Tal Afar residents -- ethnic minority Shiite Turkmen -- fled the fighting by heading north toward Iraq's Kurdish region. Tal Afar is in northwest Iraq in Nineveh province, midway between the country's border with Syria and the Iraqi city of Mosul north of Baghdad.
Violence perpetrated by ISIS shook a small town near Mosul again Sunday, security officials in Baghdad and the town of Tal Afar told CNN. Several mortar rounds landed on a busy area in the town west of Mosul and killed at least seven and wounded 33 people.
Ethnic Shiite Turkmen live in Tal Afar. Iraqi security forces still maintain a presence there.
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