Reuters, Baghdad :
Iraqi forces have fought their way into two more southeastern districts of Mosul but their advances are being slowed by Islamic State's tactic of using civilians for cover, a military spokesman said on Tuesday.
The United Nations said civilian casualties had streamed into nearby hospitals in the last two weeks as fighting intensified in the jihadist militia's last major stronghold in Iraq.
Advances by elite forces in the city's east and northeast have picked up speed in a new push since the turn of the year, and U.S.-backed forces have for the first time reached the Tigris river, which bisects the city.
But fighting in neighborhoods in the southeast has been tougher.
"The challenge is that they (IS) are hiding among civilian families, that's why our advances are slow and very cautious," Lieutenant-Colonel Abdel Amir al-Mohammedawi, a spokesman for the rapid response units of Iraq's federal police, told Reuters.
He said rapid response units and Iraqi army units had fought their way into the Palestine and Sumer districts in the last day, but that Islamic State fighters were firing at civilians trying to flee.
"The families, when they see Iraqi forces coming, flee from the areas controlled by Daesh (Islamic State) towards the Iraqi forces, holding up white flags, and Daesh bomb them with mortars and Molotov cocktails, and also shoot at them.
"Whenever they (IS) withdraw from a district, they shell it at random, and it's heavy shelling," he said.
The United Nations' humanitarian coordination office (OCHA) said nearly 700 people had been taken to hospitals in cities in Kurdish-controlled areas outside Mosul in the last week, and more than 817 had required hospital treatment the week before that.
"Trauma casualties remain extremely high, particularly near frontline areas," it said.
The U.S.-backed operation to drive the ultra-hardline militants from Mosul began in October and has recaptured villages and towns surrounding the city, and most of Mosul's eastern half.
New tactics and better coordination have helped Iraqi forces advance faster since they launched a new phase of the operation more than 10 days ago.
The advances slowed in November and December as IS put up fierce resistance and hid among the civilian population, making it difficult for Iraqi forces to target them.
Iraqi forces battling jihadists in Mosul reached the Tigris River that divides the city Sunday, a key step and a first since the launch of a huge operation in mid-October.
The Islamic State group was on the back foot in Mosul after a week of significant gains for Iraqi forces but pressed a deadly campaign of bombings in Baghdad, where two more attacks killed 18 people.
Elite Counter-Terrorism Forces (CTS) took control of the eastern end of the southernmost bridge in Mosul, a morale-booster in a 12-week-old operation that has encountered many difficulties.
CTS forces "reached the Tigris River from the eastern (side) of the fourth bridge," Sabah al-Noman told AFP. The news was also confirmed by Iraqi army Staff Lieutenant General Abdulamir Yarallah.
Tens of thousands of Iraqi forces launched an offensive on October 17 to retake Mosul, the last major urban centre in Iraq still controlled by the group that seized around a third of the country in 2014.
Several areas around the city, Iraq's second largest, were swiftly reconquered, but the elite forces that pushed into the streets of Mosul itself have faced stiffer than expected resistance.