Jihadist-led militants have launched a new push to seize the key Baiji oil refinery north of Baghdad, security officials said on Sunday.

The fighting broke out on Saturday evening and continued into Sunday, the sources said.

Security forces guarding the refinery had been able to repulse the attack with the backing of warplanes, an offical told DPA news agency, adding, however, that fighting was continuing to break out nearby.

Speaking on condition of anonymity, he said at least 30 militants were killed.

Militants from the IS group have made several attempts to take the facility since the jihadist fighters took over considerable swaths of land in northern Iraq in a lightning offensive in June. The refinery, some 200 km (124 miles) north of the capital, once accounted for over 50 percent of Iraq's supplies of refined oil products.

The unrest in the north has hit oil production and shipments there, but the huge southern fields and export terminals have not been affected.

German 'released'

This came as a German Sunday newspaper reported that a 27-year-old German man kidnapped by IS militants over the border in Syria around a year ago was released in June.

The unnamed man, from the eastern German state of Brandenburg, had traveled to Syria in June 2013 with the aim of offering "humanitarian aid," the Welt am Sonntag said.

After reporting him missing, his family received a video earlier this year containing a ransom demand and also showing the execution of another hostage, the paper reported.

The German authorities, including the foreign ministry, had subsequently entered into negotiations with the kidnappers, according to the paper.

The paper said that a substantial amount of ransom had been paid for him to be freed, basing its claims on unnamed security sources. However, it quoted the German foreign ministry as denying that payments had been made.

The Welt am Sonntag report comes just days after IS militants released a video showing the beheading of US freelance journalist James Foley, kidnapped in Syria in November 2012.

IS, formerly known as the Islamic State in Iraq and Syria (ISIS), now holds large amounts of territory in both countries, declaring a "caliphate" in the regions it controls.

tj/ksb (AFP, dpa)