More than 200,000 people have fled west Mosul since the operation to oust the Islamic State group began last month, and officials and witnesses say that air strikes have taken a devastating toll on civilians who remained in the city.
"Just arrived in Iraq to focus on the dire humanitarian situation on the ground. Protection of civilians must be the absolute priority," the UN chief said on his official Twitter account.
After his arrival in Baghdad, Guterres met President Fuad Masum, parliament speaker Salim al-Juburi and Foreign Minister Ibrahim al-Jaafari.
He was also due to hold talks with Prime Minister Haider al-Abadi before flying to Arbil, the capital of Iraq's autonomous Kurdish region.
Guterres's visit comes at a critical time for Iraq, which is fighting to retake Mosul in a battle that has sparked myriad humanitarian concerns.
Jaafari highlighted the issue of reconstruction in talks with Guterres -- a major challenge in areas that have been devastated by heavy fighting to retake them from IS.
"Iraq needs a plan similar to the Marshall Plan... to present assistance to Iraqis and support development and overcome the effect of war against (IS) terrorist gangs," Jaafari said, according to his office.
The Marshall Plan was a major US effort to help Western Europe recover from the devastation it suffered in World War II.
Widespread displacement is another issue, with Iraqi authorities saying that more than 200,000 people have fled west Mosul since February.
Camps have been set up around the city to provide shelter for the displaced, while others are staying with relatives, renting accommodation or residing in makeshift shelters or unfinished buildings.
Displacement from Mosul has not reached the worst-case scenario of a million or more people that had been feared, but that has come at the cost of huge numbers of civilians being trapped in the middle of the battle.
Civilians killed in Mosul
The UN said earlier this month that some 600,000 civilians were still in west Mosul, 400,000 of them trapped in siege-like conditions in the Old City.
Remaining in the city has posed deadly danger to residents -- the UN human rights office said that more than 300 civilians were killed in west Mosul in a little over a month.
Gunfire, shelling, bombs and air strikes have all taken their toll.
The US-led coalition carrying out strikes against IS said it had "probably" played a role in civilian deaths in west Mosul, while the Iraqi government has sought to blame the casualties on the jihadists.
Both the UN and Amnesty International have called on Iraqi forces and the coalition to do more to protect civilians in Mosul.
Amnesty's Donatella Rovera said field research in east Mosul -- which was recaptured from IS in January -- showed "an alarming pattern of US-led coalition air strikes which have destroyed whole houses with entire families inside".
"The high civilian toll suggests that coalition forces... have failed to take adequate precautions to prevent civilian deaths, in flagrant violation of international humanitarian law," she said.
IS overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led air strikes have since regained much of the territory they lost.
The jihadists still pose a threat, however, even in the capital, where a suicide truck bombing claimed by IS killed at least 14 people at the main southern entrance to the city on Wednesday evening.
Iraqi forces launched a major operation to retake Mosul in October, retaking its eastern side before setting their sites on the smaller but more densely populated west.
The fighting has inflicted heavy casualties on the Iraqi security forces, according to the head of US Central Command, General Joseph Votel.
Votel told a congressional committee that 490 Iraqi security personnel were killed and more than 3,000 wounded in the battle for east Mosul, while 284 have been killed and more than 1,600 wounded in fighting for the west.