The report also says casualties from suicide attacks increased by 17 percent, to a record high.
Danielle Bell, director of human rights of the United Nations Assistance Mission in Afghanistan, speaking Thursday described the nine percent reduction in civilian casualties a year ago as an "important step" towards minimizing harm to non-combatant Afghans.
Yamamoto expressed deep concerns at the high number of casualties caused by suicide bombings and other attacks using improvised explosive devices (IEDs).
"The report attributes two-thirds of responsibility for civilian casualties to anti-government elements, mainly Taliban and Daesh (Islamic State)".
However pro-government forces, including global troops, were responsible for 20 percent of the civilian casualties - a seven percent increase from 2016.
"The people of Afghanistan, year after year, continue to live in insecurity and fear, while those responsible for ending lives and blighting lives escape punishment", he said in a statement.
"Such attacks are prohibited under global humanitarian law and are likely, in most cases, to constitute war crimes". Such attacks killed 202 people and injured another 297 during 38 attacks last year, or about three times as many as during the previous year.
Afghan President Mohammad Ashraf Ghani said during a press conference on Sunday that the Taliban-claimed January 27 attack near a public hospital, in which over 100 people got killed, was "Afghanistan's 9/11". They claimed both attacks.
Despite the decline in overall figures, Danielle Bell, UNAMA's human right director, said, "Much more needs to be done".More news: Mystery NH Powerball victor asks to take winnings before court rules
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Out of 10,453 casualties in 2017, the report said, 3,438 were killed and 7,015 others injured.
"It has indeed been a hard year".
The letter has mentioned that the continuance of war and remaining of USA forces in Afghanistan is not in favor of anyone and that this "threatens the stability of the world".
Women and children continued to bear the brunt of the armed conflict.
The UNAMA documented that, in 2017, 359 women were killed - a rise of five percent - and 865 injured.
Bell said a key finding of the report is that civilian casualties attributed to Afghan forces decreased by 23 percent compared with 2016 due to efforts made to reduce harm during ground engagements with insurgents.
"We can not sleep day and night due to the frightening sounds of firing", an 11-year-old girl injured by a bullet during a ground engagement in Arghandab district, Zabul province in September, told UNAMA.
The UNAMA also noted that the number of airstrikes carried out by global military forces and the Afghan air force grew significantly in 2017 which had an impact on the number of casualties.