Among the top diplomats to express concerns about a possible break-up of Iraq was German Foreign Minister Frank-Walter Steinmeier.

Speaking to lawmakers in a debate in the Bundestag lower house of parliament late on Wednesday, Steinmeier reiterated a call for Iraqi Prime Minister Nuri al-Maliki to form a national unity government in which "all regions and religions are represented."

Steinmeier also said that with the Sunni Islamic State of Syria and the Levant (ISIS) continuing its advance towards Baghdad, and with Shiite Prime Minister al-Maliki unwilling to share power with Sunni and Kurd politicians, Iraq was in danger of breaking up. He also called on regional powers to play a constructive role.

"We must make it clear to neighboring countries that a failure of the authority of the state is in nobody's interest," he said during a debate about the foreign office's budget.

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Steinmeier echoed the concerns of US Secretary of State John Kerry on a day when American officials confirmed that their intelligence had confirmed that reports that Syrian warplanes had bombed ISIS positions inside Iraq appeared to be true.

Syrian government forces are also confronting ISIS fighters within the country's own borders.

"We've made it clear to everyone in the region that we don't need anything to take place that might exacerbate the sectarian divisions that are already at a heightened level of tension," Kerry said at a meeting in Brussels of diplomats from members of the Western military alliance NATO on Wednesday.

"It's already important that nothing take place that contributes to the extremism or could act as a flash point with respect to the sectarian divide," he added.

Kerry had traveled to Brussels from Baghdad following talks in Baghdad, where he called on Maliki to form an "inclusive government" as a way of defusing the crisis.

Maliki,though rejected the idea.

"The call to form a national salvation government constitutes a coup against the constitution and the political process," he said in an address broadcast on national television.

This came despite the fact that government forces have appeared to be unable to halt a two-week old advance of ISIS fighters from the north.

Also on Wednesday, it was reported tha almost half of the 300 US military advisors offered by President Barack Obama to support the Iraqi government had arrived in the country. However, he has ruled out sending ground troops to the country, which it invaded in 2003. The last US troops pulled out in 2011.

pfd/kms (AP, Reuters, dpa)