NORTH OF MOSUL, Iraq (Reuters) - In the morning, spirits were high among Iraqi troops battling Islamic State for control of Mosul as they advanced on the northern edge of the city, helped by a salvo of rockets fired by the U.S.-led coalition.
But as Friday wore on, the mood grew tense on the rooftop behind the frontline where Iraqi commanders and U.S. advisers were coordinating the fighting, as they came up against the challenges of combat in an urban environment and the militants detonated a car bomb.
"It's that time of day," said an American adviser as his Iraqi counterparts rushed to make contact with their men on the ground via walkie-talkie following the blast.
Vastly outnumbered and overpowered, Islamic State militants have adopted the strategy of waiting for Iraqi forces to reach their target before launching a counterattack when their enemy is worn out after a day's fighting.
The view from the rooftop several kilometers from the battle zone provided evidence of that pattern, and allowed a glimpse into the relationship between Iraqi commanders and their American partners.
Iraqi forces began their assault on Mosul's Hadba apartment complex early on Friday, breaching the city's northern limits for the first time since the campaign to retake the jihadists' last major stronghold in Iraq began nearly three months ago.
Pressing their advance on Saturday, troops closed in on the Tigris river that runs through the middle of Mosul. [L5N1EX04S]
Elite counter-terrorism services (CTS) pushed into the city from the east in October, but regular army units like the 16th division deployed to the north made slower progress and the offensive stalled.
Iraqi forces renewed their assault just over a week ago and have since made rapid progress in Mosul's eastern districts with increased support from U.S. forces now visible very close to the front lines.
Although the U.S. presence now is far smaller and more discreet than it was after the invasion of Iraq in 2003, its impact is clearly significant.
"Didn't the Americans tell you yesterday where the un-mined roads are?" Ali al-Freiji, the commander of the northern front, yelled over a walkie-talkie at one of his officers on the ground in the early stages of Friday's assault.
"The longer you take, the more the enemy will reinforce. The goal is to exploit the enemy's weakness."
U.S. servicemen on the edge of the roof squinted through binoculars at Mosul, over which the Iraqi flag could be seen flying in the foreground, and further away -- but still bigger -- the black banner of Islamic State.
Helicopters buzzed overhead as the Iraqi commanders directed their forces on the ground, and the U.S. adviser informed them the coalition was preparing to fire 24 long-range HIMAR rockets from a base in Qayara, south of Mosul.
"When the 24th rocket has hit, advance as quickly as possible towards the target," Freiji instructed a commander on the ground via walkie-talkie.
"Received, received," came a voice from the other end.
Then they waited for the rockets, which struck their target in quick succession, sending a thick cloud of dust and debris into the air: "That is a gift from the U.S. Special Forces," said Major General Najm al-Jubbouri.
As well as around 5,260 U.S. troops currently deployed in Iraq, there are around 100 special operations forces who conduct secret raids against senior Islamic State leaders.
The rockets paved the way for Iraqi forces to advance into the apartment complex, winning another small victory in the largest military campaign the country has seen since the U.S.-led invasion more than a decade ago.
And then began the no less challenging task of securing the gains against a counter-attack before night fell.
"Hurry up, hurry up. You've got less than an hour until sunset," an Iraqi officer urged over his walkie-talkie.
editing by John Stonestreet