By defence correspondent Jonathan Marcus in Munich

Nato's Secretary General, Lord Robertson, has stressed Nato must continue to modernise its armed forces to meet the challenges of the 21st century.

He was speaking at a European Security Conference in Munich Saturday, where he stressed technological change meant that some member countries' armed forces risked being left behind.

He also insisted Europe as a whole had to shoulder more of the financial burden of its defence.

Mixed report

Lord Robertson was reviewing Nato's performance in the Kosovo crisis.

He drew up a mixed report card from Nato.

He said the alliance undoubtedly won in Kosovo but that the crisis had provided a test of how Nato's military forces had evolved in the years since the Cold War ended.

In general, he asserted, they pass with flying colours, but he said that Kosovo had also revealed that the armed forces of some alliance members had not made all the adjustments necessary to be effective in a modern security environment.

He emphasised the need for highly mobile forces, capable of sustaining themselves beyond the alliance's own territory for lengthy periods, perhaps for years.

Lord Robertson also drew a second major lesson from the Kosovo experience, the need for a more equitable division of labour within Nato.

He told the conference: "It's neither fair nor politically sustainable to ask the United States to continue to assume a disproportionate share of the cost and the burdens of addressing security challenges in Europe.

"In the 21st Century, the European allies will have to play a stronger role in partnership with North America."

Stronger Europe

Lord Robertson stressed that a stronger Europe made sense on both sides of the Atlantic.

To say that it would lead to damaging divisions in Nato, he said, was simply nonsense.

It wasn't a question of necessarily spending more money on defence, but of getting a much better return on what was already invested.