Islamic State fighters in Iraq recaptured about one half of the town of Dhuluiya, one day after it was won by Iraqi forces, and attacked a neighbouring town just 70 km (45 miles) north of Baghdad, police officers and witnesses said on Sunday.
The radical Sunni militants have seized large chunks of territory in Iraq since the beginning of the year, first in western Iraq and after June across the country's north, imposing strict Islamic rule and forcing thousands to flee.
A stalemate exists in the country, with territory regularly switching hands between the Iraqi govermment and Islamic State.
On Saturday, Islamic State stormed a town in Iraq's western Anbar province and seized the town of Kubaisa. In Syria, Islamic State forces shelled the border town of Kobani.
Police said militants launched "well-organised assaults" on Saturday on Dhuluiya and took back control of a large part of the northern half of the town, which lies on the Tigris and is the base for the Sunni Muslim al-Jubouri tribe who are fighting Islamic State.
The attacks forced Iraqi security forces to retreat to the southern part of Dhuluiya. At least two tribal fighters were killed, seven policemen were wounded and four police vehicles were destroyed, a police officer and a witness said.
Islamic State militants also attacked the neighbouring Shi'ite town of Balad late on Saturday. Balad's inhabitants have joined the Jubouri tribe in battling the group - a rare case of a Sunni-Shi'ite alliance against the jihadists.
At least two Shi'ite fighters were killed and five others wounded in Balad when militants driving pickup trucks had a two-hour firefight with military and militia checkpoints late Saturday before retreating, police and medical officials said.
One police officer and witnesses said they believed the attack on Balad was meant to distract Shi'ite volunteer fighters from helping in Dhuluiya.
"We are fighting devils and ghost fighters who retreat one day and return back stronger on the next day," said Abu Ali al-Lami, a Shi'ite militia fighter in Dhuluiya.