MOSUL, Iraq — Iraqi special forces launched a two-pronged assault deeper into Mosul’s urban center on Friday, unleashing the most intense street battles against Islamic State militants since the offensive began nearly three weeks ago.

Smoke rose across eastern neighborhoods of Iraq’s second-largest city as heavy fighting continued after sundown, with explosions and machine gun fire echoing in the streets as mosques called for evening prayer.

More than 3,000 Iraqi troops took part in the assault under heavy U.S.-led coalition air support, but the pace of the fight also slowed as Iraqi forces moved from fighting in more rural areas with few civilians to the tight, narrow streets of Mosul proper. Sniper fire repeatedly stalled the advance, as commanders called in airstrikes or artillery support after coming under fire.

As the operation got underway, columns of armored vehicles wound through the desert, pushing through dirt berms and drawing heavy fire as they closed in on the middle-class Tahrir and Zahara districts. The area was once named after former Iraqi dictator Saddam Hussein.

Seven suicide attackers in explosives-laden vehicles barreled toward the troops, with two getting through and detonating their charges, Lt. Col. Muhanad al-Timimi told The Associated Press. The others were destroyed, including a bulldozer that was hit by an airstrike from the U.S.-led coalition supporting the offensive.

At least seven special forces troops were killed and an officer and three soldiers were wounded, said an Iraqi military officer who spoke on condition of anonymity because he was not permitted to brief reporters.

“The operation is going well, but it’s slow. These kinds of advances are always slow,” said Iraqi special forces Capt. Malik Hameed, as Islamic State fighters could be seen running in the distance to reposition themselves. “If we tried to go any faster we would take even more injuries.”