US-led coalition warplanes have dropped leaflets showing dead jihadists into Islamic State’s Syrian stronghold and warned those below that “freedom will come”, activists claim.

Warplanes also carried out 16 air strikes in Iraq and nine in Syria over the weekend, the US military said. The missions were part of a sustained campaign that has seen heavy strikes conducted near five Syrian cities, including five near Raqqa – Isis’s de-facto capital - and four near Hasaka in north-eastern Syria.

Raqqa is Being Silently Slaughtered, the anti-Isis activist group operating in Raqqa, and the Syrian Observatory for Human Rights said the leaflets contained drawings showing dead extremists and their flag turned upside down.

Four fighters with the main Kurdish militia YPG, walked down a street in the picture, with two words in Arabic below translated as "Freedom will come".

There was no immediate response from the US government about the leafleting campaign, but the tactic is often used to try and claw a psychological advantage over an enemy or spread awareness to civilians in a warzone. Previous leaflets distributed by the coalition have included cartoons showing masked Isis fighters at a “hiring office” feeding people into a meat grinder.

Western fighters join Kurds in battle against Isis Show all 9 1 /9 Western fighters join Kurds in battle against Isis Western fighters join Kurds in battle against Isis Foreign Fighters A 67-year old foreign fighter from Canada, nick-named by Kurdish fighters as Hewal Zinar, poses for a photo on April 19, 2015 Uygar Onder Simsek / AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 23-year old fighter and ex-US marine from the US, nick-named by Kurdish fighters as Hewal Agir, guards a look out point during clashes with Islamic State (IS) group fighters on April 17, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 26-year old from the US, nick-named by Kurdish fighters as Hewal Dilsad, shows a picture of his wife and son as he poses for a photo on April 14, 2015, in the outskirts of the north-western Syrian town of Tal Tamr Western fighters join Kurds in battle against Isis Foreign Fighters Ex-US marines aged 23 and 24 from the US, nick-named by Kurdish fighters as Hewal Agir (R) and Hewal Agit, guard a position during clashes with Islamic State (IS) group fighters on April 17, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A group of international foreign fighters pose for a photo on April 16, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A foreign fighter poses for a photo with a machine gun on April 19, 2015, in the outskirts of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 28-year old foreign fighter from the UK, nick-named by Kurdish fighters as Hewal Baran, poses for a photo on April 19, 2015, in the outskirts of the north-western Syrian town of Tal Tamr, north of Hasakeh AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A 21-year old from the UK and 23 year-old from the US nick-named by Kurdish fighters as Hewal Sores (L) and Hewal Agir guard a post on April 19, 2015 AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A 67-year old man from Canada and a 40-year old from the UK, nick-named by Kurdish fighters as Hewal Zinar (L) and Hewal Cudi train on April 19, 2015, in the outskirts of the Tal Tamr AFP

The development came as David Cameron gave another hint that he could soon seek Parliament's approval to extend UK military air strikes from Iraq into Syria, telling a US television network he wanted Britain to "step up and do more" to "destroy this caliphate". In remarks made to NBC News, Mr Cameron said Britain must do more fight the group.

“We know that we have to defeat Isil (Isis), we have to destroy this caliphate, whether it is in Iraq or in Syria," he said. "That is a key part of defeating this terrorist scourge that we face."

But concerns have been raised after the revelation that RAF pilots embedded with US forces took part in bombing raids over Syria despite MPs having voted against Britain carrying out strikes in the country. Defence Secretary Michael Fallon is expected to face hostile questioning in the Commons over the non-disclosure of the missions which Downing Street said Mr Cameron was aware of.