A powerful jihadist group pushing toward Baghdad is waging a parallel war online, flooding social media with updates and riding the football World Cup frenzy to spread its message with corporate-like sophistication.

As militants, led by jihadists from the Islamic State of Iraq and the Levant (ISIL), advanced on the ground in Iraq, they also invaded microblogging site Twitter, where users posted a near-constant stream of updates and photos.

ISIL puts "not just other insurgent groups to shame, but even legitimate companies that are trying to sell products online," said Aaron Zelin of the Washington Institute for Near East Policy said.

"They are very, very good."When Iraqi security forces wilted in the face of the initial militant onslaught, users posted pictures of captured military vehicles and positions on Twitter, as well as short accounts of attacks.

After the ISIL-led militants seized most of the northern province of Nineveh, photos of it bulldozing the berm dividing Iraq from Syria, symbolising the unification of the two countries, appeared online as well.

And when ISIL said it executed Iraqi security forces members in Salaheddin province, images of militants firing on scores of men, who lay face-down in shallow ditches as blood pooled in the sand, were posted on Twitter and elsewhere online.

The group has also taken advantage of the massive international focus on the World Cup to spread its content on Twitter during the offensive, which has overrun major areas of five provinces and reached to within 100 kilometres (60 miles) of Baghdad.

Major events are often given hashtags such as "#WorldCup2014" that allow Twitter users to easily search for related content.

The group has hijacked World Cup hashtags in English and Arabic to share pro-ISIL content, in addition to using various ISIL-specific hashtags as well.

- Fusing 'quantity and quality' -

"ISIL appears to be fusing both quantity and quality increasingly effectively," said Charles Lister, a visiting fellow at the Brookings Doha Centre.

"The constant flow of material and its high quality provides followers with the image of a highly organised, well-equipped organisation seemingly (worthy) of joining," he said.

"For this reason, countering this propaganda material should arguably be seen as being as important as stemming the intensity of conflict in the region."

For militant groups, the fight over public perception can be even more important than actual combat, turning military losses into propaganda victories and battlefield successes into powerful tools to build support for the cause.

"They have a really smart plan. All jihadi groups are very good at what they do, but (ISIL) really stands out," said Zelin.

"They have been targeting their messages to people of different languages using popular hashtags (and) they have also created their own (application) for Twitter," he said.

Zelin said the application, which has since been discontinued, would send out the same message on all linked Twitter accounts, "so it would flood things".

"Therefore, they would be able to own a particular issue or message or topic they were trying to push out there," he said.

The fate of the application, however, illustrates a problem faced by jihadists on Twitter and other social media sites that have rules under which users advocating violence or posting other objectionable content may be suspended or banned.

When the offensive began late on June 9, there were ISIL-affiliated Twitter accounts dedicated to various "wilayas," or states, into which the group divides Iraq.

But some accounts covering areas where the fighting took place were suspended by Twitter as the offensive progressed.

For ISIL, "the question will be, how do they react and evolve or be able to continue to communicate to a broader audience," Zelin said.

Nathaniel Rabkin, the managing editor of the Inside Iraqi Politics newsletter, said that ISIL is also carrying out on-the-ground propaganda efforts in Iraq, such as "holding mass 'repentance' gatherings, where policemen, soldiers, and Sahwa (militia) men pledge to stop working with the government."

"I suspect these street theatre type of events are more important avenues of propaganda for (ISIL) inside Iraq than the videos posted online, which may be more directed at an international audience," he said.