WASHINGTON — An exhausted and ill-equipped Iraqi Army faces daunting obstacles on the battlefield that will most likely delay for months a long-planned major offensive on the Islamic State stronghold of Mosul, American and allied officials say.

The delay is expected despite American efforts to keep Iraq’s creaky war machine on track. Although President Obama vowed to end the United States’ role in the war in Iraq, in the last two years the American military has increasingly provided logistics to prop up the Iraqi military, which has struggled to move basics like food, water and ammunition to its troops.

Without the help, Americans commanders said, the offensive against Mosul would most likely fail.

Americans are ferrying equipment and spare parts directly to the battlefield by cargo plane, helping arrange purchases of ammunition for Soviet-era equipment and pressing the Iraqis to adopt measures to improve a supply chain that would run over 200 miles from Defense Ministry depots in the Baghdad area to Mosul.

But no matter how hard the Americans push, the Iraqis can go only so fast. The pace of ground operations is likely to become even slower in the summer’s searing heat and during the coming holy month of Ramadan, when Muslims often fast during the day. Much of the Iraqis’ equipment needs to be repaired or replaced, and many Iraqi units will require additional training before attacking Mosul.