Mr Claes, who took office last year after the death of Manfred Worner, believes the alliance has yet to come to grips fully with the collapse of the Warsaw Pact. The threat from the Soviet Union had been " a clear challenge, which could be situated in time and on the map," he told the Independent.
"Now we are facing other risks, which are much more difficult to situate in a clear way." Prime among these, according to the former Belgian foreign minister, are the rise of Islamic fundamentalism in North Africa and security arrangements for the formerEastern Bloc states.
The alliance faced its first test in Bosnia, with uncertain results. If the US Congress goes ahead with its threat to lift the arms embargo on the Bosnian government, the results could be catastrophic. "There is still a risk of a serious clash if the US takes the decision on a unilateral lifting of the arms embargo."
Nato is already preparing for the possible evacuation of UN peace-keepers from Bosnia. And the alliance is also considering how it could withdraw peace-keepers from Croatia if President Franjo Tudjman carries out his threat to end their mandate, Nato sources said.
But Mr Claes was optimistic: "I still believe that there are a lot of possibilities to convince President Tudjman of the necessity to reconsider his position."
However, he added: "It is our job to prepare for everything." Nato sources say the alliance has already been sounding out the Croatian authorities on the use of ports, roads and airfields.
The alliance is also looking at what might happen if there is an extension of the conflict, and a need to contain it from spreading through the Balkans. But the old animosities between Nato and the UN have resurfaced. "Once again we have clearly put thison the table, and it's up to the UN," he said. A Nato delegation visited New York to discuss it, but little has happened.
Nato was carrying on with its study on the consequences of bringing in new members from the east, with meetings taking place every week. Differences between the US and Europe over this had been overblown. "I did not see such big differences over enlargement. I do not pretend that there are not nuances between the 16."
Enlargement also carries with it the risk of a future Nato split, Mr Claes acknowledged. "One cannot exclude that we will face some difficulties when we start the second stage of the process, namely the who and when."
The study's completion will be followed by a "global report" to ministers in December, but Mr Claes would not speculate on whether this would immediately lead to the second stage - a specific timetable for eastern enlargement. When the study is completed, "we will inform Moscow," said Mr Claes.
Relations with Russia have been all but frozen for the past few months. Last year, Russia refused to sign two agreements with Nato; while the war in Chechenya is going on, it is unlikely that either side will want to raise these again, according to Nato sources.
In the meantime, Nato is working on Partnership for Peace, the US initiative from last year to forge new links with Eastern Europe and the states of the former Soviet Union. This week Austria becomes the latest state to sign up. Virtually every European state apart from Ireland and Switzerland is now a signatory, including Russia.
The Nato secretary-general was clearly sceptical of US calls for a new organisation to handle relations between Nato and Russia. "First things first. Let's start with what's on the table," he said, referring to the two unsigned agreements. But alliance sources say that as enlargement enters its second phase, some new arrangement - perhaps a new treaty - is inevitable. In the future, Europe has said that it will develop its own security organisation through the Western European Union. Next year's inter-governmental conference (IGC) to rewrite the Maastricht treaty will focus on this.
Mr Claes was a strong advocate of this when Belgian foreign minister and remains involved. "I have the feeling that in the IGC, the decision will be taken to strengthen the links between the EU and the WEU," he said. But he doubted that the EU itself would take over the WEU. "I do not think the time is ripe."
Perhaps the biggest challenge for any Nato secretary-general is to bridge the gap between the European and US sides of the alliance. Mr Claes warmed to the idea of new ways of linking the EU and the US, suggested by both France and Britain."I am in favour of strengthening and giving more diversity to these links," he said.
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