Top officials from the US-led coalition against the Islamic State group met in London on Thursday for the first talks since the Paris attacks with the threat posed by homegrown jihadists high on the agenda.

US Secretary of State John Kerry and British Foreign Secretary Philip Hammond co-hosted discussions involving 21 of some 60 countries that are working together to tackle the militant IS group in Syria and Iraq.

One of the three Paris attackers trained with Al-Qaeda's Yemeni branch, according to Yemeni intelligence. Another said he was inspired by Islamic State jihadists.

Kerry said taming IS was "the challenge of our time".

"Their goal is to suppress and to take over and to expand a very nihilistic, unbelievably oppressive sense of how people ought to live," Kerry said.

"We've seen them carry it out in the most egregiously horrendous fashion with public beheadings, they're now threatening two Japanese hostages.

"It's the challenge of our time. And we need to step up and lead," he added.

The US official also said that members of the coalition would now meet on a monthly basis, but not necessarily at a ministerial level.

A US state department official said jihadists leaving home to fight with IS would be a "real focus" of the meeting and that an expert working group would be formed on sharing information to stop militants travelling.

The Paris attacks rekindled fears about the dangers posed by well-trained homegrown jihadists returning from foreign battlefields.

European police agency Europol estimates up to 5,000 EU citizens have gone to join the ranks of militants in Syria and Iraq.

Meanwhile, Belgian authorities on Wednesday charged a sixth person with terrorism following a series of raids in Belgium last week that foiled a plot to kill police.

- 'Race against time' -

Looming over the meeting is also the deadline set by members of the IS group for Tokyo to pay a $200 million ransom for the release of two Japanese hostages. Tokyo believes the deadline will expire at 2:50 pm (0550 GMT) on Friday.

Japanese Foreign Minister Fumio Kishida will not be at the London meeting, but held talks with Hammond on Wednesday as Japanese Prime Minister Shinzo Abe admitted it was a "race against time" to free the men.

Speaking to the BBC before the talks, Hammond warned that the Iraqi army might be months away from mounting a sustained fightback against IS.

"We are renewing and regenerating the Iraqi security forces -- re-equipping them, retraining them, reorganising them -- but it will be months yet before they are ready to start significant combat operations against (IS)," he said.

But he stressed that the conference was "about the other strands of this campaign" and not just military operations in Iraq.

"We're very clear that undermining the narrative of ISIL, interdicting the flow of foreign fighters, stopping the flow of financial funding to ISIL, is as important as the military campaign itself," he said.

Across town in Downing Street, Prime Minister David Cameron also held talks with Iraqi counterpart Haider al-Abadi.

Abadi defended his country's efforts to repel IS, saying the "Iraqi people have sacrificed their lives" to halt its advance.

"We have reversed, some time ago, the advances of Daesh (IS) and we are very keen to push them back from the whole of Iraq," he told Cameron.

Ministers will also discuss military efforts to support both Iraqi and Kurdish armed forces, how to cut IS financing, and supply aid for those caught in the crossfire.

The countries confirmed to be at Thursday's conference are Australia, Bahrain, Belgium, Canada, Denmark, Egypt, France, Germany, Iraq, Italy, Jordan, Kuwait, the Netherlands, Norway, Qatar, Saudi Arabia, Spain, Turkey and the United Arab Emirates.

The coalition last met in Brussels in December.