The UN chief has offered support for Iraq's fight against Islamic State during a visit to Baghdad, even as the country faces a growing humanitarian crisis following the months-long battle for Mosul.
Antonio Guterres expressed support for Iraq's fight, condemned IS crimes against civilians and pledged continued aid for the some three million Iraqis displaced by the fighting.
At a joint press conference with the UN secretary-general, Iraqi Prime Minister Haider Abadi said Iraqi forces were doing their "utmost" to protect civilians in the fight against IS and accused the extremists of intentionally putting civilians in harm's way while also "intensifying their propaganda machine to spread rumours".
Hours earlier, as Mr Guterres arrived in the Iraqi capital, IS claimed responsibility for a suicide truck bombing in Baghdad the previous night that killed 15 people and wounded 45.
The fight to take back Mosul, Iraq's second-largest city, began in October, backed by the US-led international coalition.
After routing IS from the eastern part of the city, Iraqi forces in February launched a push to drive the militants from the western half.
The Tigris River separates Mosul into its eastern and western sector.
Shortly after landing at Baghdad International Airport, Mr Guterres said on Twitter that his visit is to "focus on the dire humanitarian situation on the ground".
Amid reports of dozens of civilians killed by air strikes in Mosul, he added that "protection of civilians must be the absolute priority".
Mr Guterres met Iraq's president and parliament speaker as well as the prime minister.
The UN chief also plans to meet leaders in Iraq's northern semi-autonomous Kurdish region.
Since the start of the Mosul military operation, more than 350,000 people have fled the fighting, according to UN figures.
On Thursday, Iraq's militarised federal police inched deeper into Mosul's old city - a dense urban core in the city's western half - where they met with stiff resistance from IS militants.
First Lt Walid Khalid, of the 3rd brigade Federal Police, said his troops advanced about 100 metres in the last two days.
"The distance between IS and us is 50 metres, yesterday, we killed four IS fighters and right now their bodies are in the street," Khalid said.
"The situation is very good and the Iraqi air force is doing well."
He said the troops were about 100 metres from a symbolic mosque where the IS leader, Abu Bakr al-Baghdadi, declared the extremists' self-styled caliphate in the summer of 2014.
Also on Thursday, the SITE Intelligence Group, which monitors extremist groups, reported the IS claim of responsibility for the bombing late on Wednesday in Baghdad.
In the claim, IS warns Iraqi Shiites that the "flame of the battle" in Mosul will come to them in the cities of Baghdad, Karbala and Najaf.
In the attack, a suicide truck bomb targeted a police checkpoint on the Iraqi capital's main southern entrance.
Three policemen were among the 15 dead while the rest were civilians, police and health officials said.
The militants have suffered a string of defeats over the past two years in the lead-up to the Mosul operation, but have continued to regularly launch attacks in and around Baghdad.
A series of large-scale bombings claimed by IS has also struck Baghdad since the operation to retake Mosul began.
Iraqi and coalition officials have repeatedly warned that after Mosul, IS is likely to return to its insurgent roots as it loses more territory in both Iraq and neighbouring Syria.