Mr Kerry's knowledge of French prompted scorn from the right-wing US press during his unsuccessful bid for the presidency in 2004, due to the souring of US-French ties over the war in Iraq.

Since then, he has played down his language proficiency, bolstered during childhood summers spent at his mother's family estate in Brittany.

"Big heels"

Foreign Minister John Baird was the first foreign dignitary to be welcomed to the state department by Mr Kerry since he took office last week.

Speaking alongside Mr Baird at his maiden press conference as the top US diplomat, Mr Kerry urged Iran to address concerns about its nuclear programme.

The 69-year-old former Massachusetts senator said the US was committed to preventing Iran from securing a nuclear weapon and would continue its policy of both pressure and engagement to try to bring this about.

He said the international community would respond, if - as he put it - Iran was ready to talk substance at the negotiations to take place in Kazakhstan later this month. Iran insists its nuclear programme is solely for civilian purposes.

The 2004 Democratic presidential candidate was greeted by a cheering crowd of employees on Monday in the large black-marble lobby of the state department.

He succeeds Mrs Clinton who, in her four years as the top US diplomat, visited 112 countries and notched up nearly a million air miles.

In a speech to staff, Mr Kerry joked that he had "big heels to fill".

The new state department head has spent much of his first week getting in touch with foreign leaders.

Last weekend, he spoke to Israeli Prime Minister Benjamin Netanyahu and Palestinian Authority President Mahmoud Abbas.

Mr Kerry also contacted the foreign ministers of Japan, South Korea, Turkey and Mexico, as well as Mr Baird.