KABUL (AFP) - The Taleban warned Kabul residents on Monday (May 21) to avoid "military centres" in the heavily fortified city, saying they are planning more attacks in the capital, where civilians have long formed the bulk of the casualties.

The militant group has issued such warnings to civilians before, including during a failed attempt to take the western city of Farah last week, but it is believed to be the first time they have singled out Kabul.

The warning comes after the United Nations said the war-weary capital - where the Islamic State in Iraq and Syria (ISIS) group is also stepping up its attacks - is already the deadliest place in the country for civilians.

The Taleban said they are planning more attacks on "the enemy's military and intelligence centres" as part of an annual spring offensive.

"Therefore, to avoid civilian casualties and only cause damage to enemy military, we are asking Kabul residents to keep away... We don't want even a single innocent civilian to be killed," a statement published online said.

The group did not define what was meant by "military and intelligence centres".

Such targets are difficult to avoid given the overcrowded city is the heart of the country's intelligence, government and military operations and also plagued by traffic jams due to ubiquitous checkpoints and barriers.

"Any attacks or explosions, even a small one, would cause civilian casualties because military installations are located in the centre of the city near people's houses," political and military analyst Nik Mohammad told AFP.

The Taleban's statement was pure propaganda, he said, adding that if they fight in the cities, "you will definitely kill civilians, there is no way to avoid that."

The Taleban are stepping up their Al Khandaq spring offensive in an apparent rejection of calls for the militants to take up the Afghan government's February offer of peace talks.

The group portrays itself as taking care to avoid civilian casualties, but has claimed attacks such as a massive bomb hidden in an ambulance in January which detonated in a crowded street and killed more than 100 people.

The extremists' chilling ability to hit at the heart of the country despite increased police checks has spotlighted security and intelligence failures, with the government of President Ashraf Ghani coming under increasing pressure to protect civilians.

Kabul - overflowing with returning refugees and internally displaced Afghans fleeing war and seeking jobs and security - has been the deadliest place in the country for civilians for months.

Figures from the United Nations Assistance Mission in Afghanistan (UNAMA) show that Afghan civilians were deliberately targeted in militant attacks and suicide blasts in 2017.

The capital is a top target, with 16 per cent of all casualties during the year - a total of 1,831 people killed and wounded - occurring in Kabul alone.

The UN has warned that 2018 could be even deadlier.