A group calling itself "Mosul Liberation Forces" is urging Iraqi Prime Minister Haider al-Abadi to declare war against the terror group Islamic State (ISIS), saying they are tired of waiting and are ready to give support for the retaking of Mosul.

"We demand [Iraqi Prime Minister Haider al-] Abadi to declare the war against Da'esh," stated the letter, using the regional – and derogatory – term for ISIS, which has ruled the city for 18 months.

"We are tired of waiting. We will give support to liberate Mosul once you declare zero hour," it added.

It was not known who sent the letter, which was handwritten in Arabic and posted on a light pole in downtown Mosul this week, Fox News reported.

Some sources say the message could be aimed at "galvanising the increasingly restless underground resistance in the northern city, once a cultural and educational jewel in the embattled country—or, something different altogether."

Others inside the occupied city said the posted message could be a plot by the increasingly paranoid ISIS to flush out anyone in the city who supports the Iraqi government.

The development comes as Baghdad prepares to retake Iraq's second city that was captured by the terror group in 2014.

The Iraqi government earlier announced its plan to launch an offensive on Mosul backed up by the U.S-led coalition forces, Kurds and Christians.

Following its recapture of Ramadi, the Iraqi government is targeting Mosul next, dropping leaflets by air and making radio broadcasts promising liberation for the people of the city.

Reports say U.S. airstrikes on Mosul in the past weeks have taken out ISIS banks and warehouses that store the hard currency needed to keep the city running and ISIS fighters paid.

In addition, coalition warplanes have destroyed nine ISIS fighting positions, a weapons factory, and a number of weapons caches, according to an incident review report.

U.S. officials expressed belief that the ongoing air campaign in Iraq specifically in Mosul and the planned assault spearheaded by the government could bring about a protracted and bloody assault.

Talks are now underway about how the campaign in Mosul will be coordinated, sources told Fox News.

"Ramadi was a schoolyard fistfight. Mosul is the heavyweight championship bout," said retired U.S. Army Lt. Col. Ralph Peters, a Fox News Channel contributor.

Pentagon announced last week that hundreds of Western military trainers will soon be deployed to the region to help Iraqi security forces prepare for a highly-anticipated campaign.

"[Further training was] the next step in generating the amount of combat power needed to liberate Mosul," U.S. Defense Secretary Ash Carter was quoted as saying.

Mosul has an estimated population of 1.8 million. It became the largest city to be captured by ISIS, which later declared a caliphate, stretching from Aleppo, Syria to the province of Diyala, Iraq.