France's foreign minister has said his newly-appointed British counterpart, Boris Johnson, is a liar with "his back against the wall".

In comments to Europe 1 radio, Jean-Marc Ayrault said Mr Johnson had lied to the British people during the recent EU referendum campaign and would now be under pressure "to defend his country".

He said France needed a negotiating partner who was credible and reliable.

In the meantime, U.S. Secretary of State John Kerry has called Johnson to congratulate him on his new job and urge "a sensible and measured approach" to Britain's withdrawal from the European Union.

In a Thursday phone call from Paris, Kerry and Johnson "agreed that the U.S.-U.K. special relationship is as essential as ever" and pledged to work together as NATO allies in facing challenges.

The State Department said Kerry told Johnson that the U.S. would stay engaged with Britain as it moves ahead in the process of leaving the 28-nation bloc, and the two men agreed to meet next week in Brussels.

And Germany's finance minister is signaling a willingness to forget past statements by Boris Johnson, Britain's new foreign secretary.

Ahead of last month's referendum on European Union membership, Johnson compared the bloc's aims with those of Adolf Hitler. Asked what he expected of working with Johnson, given such comments, Finance Minister Wolfgang Schaeuble said — to laughter — "you see that it's an advantage not to have such a great command of the English language."

He added Thursday: "We in Germany have had good experience with putting comments made during a campaign into the file for election campaigns, and forgetting them on the day after the democratic decision has been made. So I don't know what you're talking about."

Schaeuble is influential in Germany's government and in Chancellor Angela Merkel's party.