US-led strikes against Islamic State group officials in Iraq and Syria are robbing the jihadists of one of their most valuable resources: experienced mid-level commanders.

Ten of the group's higher-ups, including one with "direct" ties to the alleged mastermind of the Paris attacks, were killed in air strikes in December alone, the US military said.

According to analysts, these deaths chip away at IS "middle-management" -- seasoned and skilled commanders within the group.

"Because of their operational role and their experience, these figures are an invaluable human resource and a huge loss for IS," said Mathieu Guidere, a jihadism expert at the University of Toulouse.

Instead of focusing on IS's political chiefs the US-led coalition has targeted "technical cadres and the mid-level commanders who, though they don't take the decisions, execute them", said Guidere.

"Without them, nothing could be done on the ground."

Most of the commanders were killed in Iraq, where Washington is working with government forces, but others were targeted in Syria.

The dead include Charaffe el Mouadan, who had ties to the Paris attacks "cell leader" Abdelhamid Abaaoud and was killed in a coalition strike in Syria on December 24, according to the Pentagon.

Yunis Khalash, IS's "deputy financial emir" in the group's Iraqi stronghold Mosul, was killed on December 9, it said.

"His death will burden senior ISIL (IS) cadres to find a technically skilled and trustworthy replacement," said US-led coalition spokesman Colonel Steve Warren.

- 'Experienced cadres' eliminated -

Also killed in Iraq earlier this month was Khalil Ahmad Ali al-Wais, IS's "emir of Kirkuk province", according to the Pentagon.

Iraqi analyst Hisham al-Hashimi said Al-Wais, known as Abu Waddah, also headed IS's internal communications and mail service.

He named several other IS operatives killed in December, including leading members of the "Hesba" -- the force that implements IS's strict religious and social code.

"On the one hand, IS has lost its operational and executive capacity. On the other hand, it has lost its ability as a structured hierarchy to work together as a team," Hashimi said.

IS has also been dealt a series of military blows on the ground in recent weeks.

Earlier this week, Iraqi forces pushed the group out of the city of Ramadi, while an alliance of Kurdish and Arab forces in Syria rolled the jihadists back across the Euphrates River.

Yezid Sayigh, an analyst at the Carnegie Middle East Centre, said IS's recent losses could be a sign that the United States is receiving better intelligence on the group's activities.

"But it may also be a result of weakening security practice among IS personnel as more experienced cadres are taken out of action," he said.

The "cumulative impact" of these deaths could be a turning point in the war against the jihadist group, he said.

"What the trend really reveals is that IS reached the full extent of its potential in 2014 and was unable to expand its resource and socio-political base or maintain high levels of mobilisation."

- 'Double-edged sword' -

Analysts say IS has benefited from its decentralised structure, but may now be forced to close its ranks, leaving it more vulnerable as it struggles to push forward.

IS "has transformed over the past year from a decentralised organisation to a highly centralised one, relying on total control, which will cause it to erode further," Hashimi said.

The jihadist group will also have difficulty finding replacements with enough institutional knowledge, added Guidere.

"It's not so much the replacement itself that is a problem, since there are plenty of candidates," he said.

"The problem is not the quantity but the quality, which is at risk of becoming rare."

The vacuum could result in more brutality, said Mia Bloom, a jihadism expert at Georgia State University.

"The strategy of divide and conquer, in which one aims to foster splinters and divisions within terrorist groups, is a double-edged sword," Bloom said.

"With the loss of a leader there are incentives for the up-and-coming middle management to distinguish themselves and this is often done in very bloody and violent ways," she said.

Candidates could "try to outbid each other with deadly attacks" with newly-promoted leaders seeking to "establish their street cred", she added.