The ambitious "Euphrates Shield" operation -- with Turkish forces backing pro-Ankara Syrian rebels in an unprecedented incursion -- began in spectacular style in August as the army ousted jihadists from a succession of border towns including Jarabulus.
But Al-Bab, which symbolically means "The Gate" in Arabic, has proved far tougher, with Turkish officials predicting repeatedly in the last few weeks that it will be taken imminently but with no clear end in sight.
At least 48 Turkish soldiers have been killed in the incursion so far, according to an AFP tally, the vast majority in the battle for Al-Bab since the fight for the town began on December 10.
President Recep Tayyip Erdogan vowed on Friday that Turkey would "finish the job" in Al-Bab, but indicated it was not necessary to push any deeper inside Syria.
Turkey has repeatedly complained of being isolated by its NATO allies in the operation, although Ankara has recently won some backing from its newfound ally Moscow.
But the operation has come with NATO's second largest standing army facing troubles after the failed July putsch, with more than 6,000 soldiers and 168 generals -- half the entire pre-coup contingent -- arrested in the crackdown.
Showing the tremors from the coup are still shaking the army, several soldiers who had been due to go on trial last week did not appear in court in Istanbul as they were waging the Al-Bab campaign.
"Euphrates Shield is under-resourced," said Aaron Stein, resident senior fellow at the Atlantic Council's Rafik Hariri Center for the Middle East.
"The rebels Turkey is fighting with are poorly trained and have, for years, proved incapable of taking and holding territory."
Whereas Jarabulus is practically on the border, Al-Bab is 25 kilometres (15 miles) south of the frontier and a far tougher logistical proposition.
Faruk Logoglu, a former Turkish ambassador to the United States and ex-opposition MP, said the Turkish-led campaign "is lacking final objectives and an exit strategy".
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"The target given is well beyond what's achievable. That's the problem," he told AFP. "Turkey risks being drawn further into the Syria quagmire."
IS in December claimed to have burned to death two Turkish soldiers -- although this was never confirmed by Ankara -- while the corpses of two kidnapped soldiers were returned this month.
'Moved forward alone'
Soner Cagaptay, director of the Turkish Research Program at The Washington Institute, said Turkey had suffered from the lack of support for the operation from the United States.
"Because Ankara launched its move to take Al-Bab from ISIS without securing concrete cooperation with the US, Turkey had to move forward alone," he told AFP.
"This naturally slowed down the operation. This is why Ankara has moved to secure Russian air support."
In November, the Pentagon said the US-led international coalition was not backing the Al-Bab campaign because it was "independently" launched by Turkey.
That prompted Ankara to turn to Moscow, even though the two countries have been on opposing sides of the Syria conflict since it erupted in 2011.
But Turkey and Russia late last year brokered a ceasefire in Syria and have stepped up cooperation since.
The two countries on January 18 staged their first joint air strikes against IS around Al-Bab, the Russian defence ministry announced.
By taking Al-Bab, Turkey is keen to prevent Syrian Kurdish militia allied to the US establishing a stronghold in the area. Ankara even wants to push northeast to Manbij, where the Kurds already ousted IS.
In January, the US-led coalition also conducted four strikes near Al-Bab and Turkey has greater expectations from the new US administration under President Donald Trump.
Cagaptay said Turkish forces were being targeted by IS foreign fighters who had been largely encircled by offensives in Syria and Iraq and were engaged in a fight to the death, ready to employ suicide bombers.
"For these foreign fighters, there are two ways out: capture by anti-ISIS forces, or death," he said.