An Islamic State spokesman has urged sympathizers in Europe and the US to launch attacks on civilians there if they are unable to travel to the group’s self-declared caliphate in Syria and Iraq.

In a 31-minute audio message released late on Saturday by the Isis media arm al-Furqan, Abu Mohammed al-Adnani told his followers: “The tiniest action you do in the heart of their land is dearer to us than the biggest action by us … There are no innocents in the heart of the lands of the crusaders.”

He encouraged lone wolf attacks during the Muslim holy month of Ramadan, which starts early in June, “to win the great award of martyrdom”.

Al-Adnani also said the US-led war against the group was doomed to failure and that America “fell in the swamp of perdition”.

The speech may have been aimed at boosting morale after the group suffered a string of military setbacks and lost territory in both Iraq and Syria.

Last week, Brett McGurk, the US presidential envoy to the 66-country anti-Isis coalition, said: “This perverse caliphate is shrinking.”

Addressing these losses, al-Adnani said that even if Isis were pushed out of its strongholds such as Raqqa and Mosul, this would not count as defeat because “defeat is the loss of will and the desire to fight”.

His mention of the possible loss of the Syrian city of Raqqa coincided with a secret trip to Syria on Saturday by Gen Joseph Votel, the new commander of US forces in the Middle East. The US is trying to develop credible Arab fighters to retake Raqqa, the self-declared Isis capital.

Mosques in Raqqa broadcast an announcement that civilians would be allowed to leave the city, after earlier this week planes thought to belong to the international coalition dropped flyers instructing residents to leave ahead of an offensive.

Al-Adnani cited Omar al-Shishani as one of the current leaders of the Isis group, indirectly denying a Pentagon report that in March an airstrike in Syria killed al-Shishani, described as the Isis “minister of war”.