NATO is not about to take a major role in Iraq, its secretary general has said.

Lord Robertson played down suggestions by American officials that the alliance could take a direct role in Iraq as America reduces its presence.

The NATO chief said on Tuesday that the alliance was at full stretch because of other commitments.

"It would strain an organisation that is already being strained by Afghanistan,'' he told the BBC.

"Things tend to happen incrementally rather than with a big bang. At the moment NATO has got one big bang to handle and that is taking over the stabilisation force in Kabul and then moving out of Kabul into the regions there."

But Lord Robertson, who is set to stand down at the end of this month, did not completely rule out NATO taking on duties in Iraq.

"It is not beyond the realms of possibility that next year at some point a bigger role could be put forward by the countries but it won't be put forward in my view until we have got Afghanistan right," he said.

With negotiations continuing on the EU constitution, he said NATO did not perceive an EU defence force as a threat.

"I don't think that that will in any way destroy NATO. It might well be unnecessary duplication,'' he said."An argument over 30 or 40 operational planners is something that has to be finessed.

"None of it is going to be an absolute danger to NATO but what we don't want is unnecessary duplication and what we cannot afford is any form of competition between NATO and the European Union."

Lord Robertson's comments received support from the Liberal Democrats.

"The alliance has a significant task to perform in Afghanistan, and to increase the burden by giving it responsibilities in Iraq would leave the organisation under great strain," said Menzies Campbell, the party's foreign affairs spokesman.

"A better alternative is to seek wider UN participation but that will only be achieved by allowing the UN greater political influence in reconstruction."