The Iraqi news site Shafaq reported that three columns of Iraqi soldiers had advanced on Tikrit on Tuesday morning but had withdrawn after coming under withering fire from militants. Islamic State forces have held the city, about 80 miles north of Baghdad, since June.
The situation near the dam and Mosul, an Islamic State stronghold, seemed stable Tuesday. "We have regrouped and we are ready to defeat the terrorists," said Maj. Gen Wahid Kovali, at a front line post about a dozen miles east of Mosul.
The battle in Tikrit, which Iraqi forces tried unsuccessfully to retake in June, suggested the Islamic State fighters would have better luck holding on to urban areas in northern Iraq, where they enjoy support among some Sunni Arab tribesmen.
U.S. military advisors in Iraq have been wary of government forces attempting a ground battle in Tikrit because the militants are believed to be well entrenched. But the town carries significance for Baghdad not only as the birthplace of Hussein but also because it lies on a strategic highway between Samarra, home to a revered Shiite Muslim shrine, and Baiji, the country's largest oil refinery.
Also Tuesday, United Nations officials in Geneva announced a major relief operation aimed at helping half a million Iraqis who have fled their homes to escape the fighting.
Planes from Jordan were expected to begin a four-day airlift bringing tents, kitchen goods and other supplies to northern Iraq, with land and sea shipments to follow in the coming days, officials said.
"Emergency support is an urgent need that we are trying to meet," an agency spokesman, Adrian Edwards, said in a statement.