The German Foreign Minister, Joschka Fischer, has warned his party, the Greens, that its criticism of the NATO air-strikes against Yugoslavia is threatening its governing coalition with Chancellor Schroeder's Social Democrats.

In a newspaper interview with Frankfurter Rundschau, Mr Fischer warned that, if the special party congress in May voted against NATO's campaign, Chancellor Schroeder would abandon the Greens and form a new coalition.

Mr Fischer was speaking the day after the Greens' executive issued a policy statement, in response to pressure from party members, calling on NATO to announce a unilateral ceasefire.

Mr Fischer said such a move would be a victory for President Milosevic. Another leading Green, the party spokeswoman, Antje Radcke, has said the party is in danger of splitting over the Kosovo conflict.