The 28 members of the NATO military alliance are concerned by cost overruns and delays at its new high-tech headquarters in Brussels, NATO secretary general Anders Fogh Rasmussen said Monday.

Rasmussen said the builders, Belgian-Dutch consortium BAM-Alliance, had asked for more money and more time beyond the current 2016 target date.

The Belgian government, which is in charge of the project, was looking at the claim in "a hard commercial negotiation," he said.

"This claim is a matter of concern to all of us," he added at the HQ which it has occupied near Brussels international airport since 1967.

A recent report in German news weekly Der Spiegel, citing diplomatic documents, said BAM-Alliance had asked at the end of last year for another 245 million euros and an extra nine months to finish the work.

NATO and Belgian government officials said they could not confirm such figures for a project which was last estimated to cost 750 million euros.

When work began in 2010, the building was supposed to cost 500 million euros and be completed by 2015.