The US has sent 130 more military advisors to the Kurdish region of northern Iraq, Defence Secretary Chuck Hagel says.
The marines and special operations forces will assess the humanitarian situation and will not be engaged in combat, a US defence official said.
The US has been carrying out air strikes against fighters from militant group Islamic State (IS).
IS fighters have forced tens of thousands to flee their homes.
"This is not a combat boots on the ground kind of operation," Hagel said, in remarks made at Camp Pendleton in California.
The "assessment team members" had arrived in the northern city of Irbil and would "give more in-depth assessment of where we can continue to help," he said.
The personnel are in addition to about 250 military advisers already in Iraq.
A US defence official said the government would continue to explore ways to support "Iraqis affected by the ongoing fighting in Sinjar", and to prevent "potential acts of genocide" by IS.
The UN has said that tens of thousands of civilians, including members of the Yazidi sect, are trapped on Sinjar mountain by IS fighters and need "life-saving assistance".
Meanwhile, a suicide bomber attacked a checkpoint near the home of newly-appointed Iraqi Prime Minister Haider al-Abadi in Baghdad, Reuters news agency reported, citing security sources and local media.
There were no immediate details on casualties.
Iraq's president asked Haider al-Abadi to form a new cabinet on Monday, snubbing the incumbent PM Nouri Maliki.
The move came after months of political infighting, which experts say has contributed to Iraq's inability to fight the IS threat.
Analysis: Jim Muir, BBC Middle East correspondent
Abadi is certainly off to a flying start, given the near-universal relief that an alternative to the contentious Maliki has finally emerged.
But he faces a gargantuan task.
Pulling the fragments of Iraq back together, and especially bringing the suspicious Sunnis back on board, is going to take a lot more than pious words and good intentions.
The UN says there are now an estimated 1.2 million internally displaced Iraqis.
The US, Britain and France have been delivering humanitarian aid to the Yazidis trapped in the north.
The US government says its planes have air-dropped nearly 100,000 meals and more than 27,000 gallons (123,000 litres) of fresh drinking water to the area, with the latest operation taking place on Tuesday.
The US has also reportedly begun supplying weapons to the Kurdish forces, known as Peshmergas, who have been fighting IS in the north.
The rapid advance across Iraq by IS militant fighters has thrown the country into chaos.
IS overran Iraq's second largest city, Mosul, in June. They had taken the central city of Falluja and parts of nearby Ramadi in December 2013.
On 29 June, IS said it had created a caliphate, or Islamic state, stretching from Aleppo in Syria to the province of Diyala in Iraq.