Hillary Clinton, the US Secretary of State, has compared the challenge of fighting climate change to her own struggle to lose weight.

Speaking to State Department staff on Earth Day, Mrs Clinton said more must be done to reduce the department's environmental footprint and conceded this was a big challenge, much like one of her personal battles.

"Often times when you face such an overwhelming challenge as global climate change, it can be somewhat daunting - it's kind of like trying to lose weight, which I know something about," she said to laughter.

"You think, oh I only have to lose X numbers of pounds but it seems like such a far away goal," she added.

"It's kind of like world peace and so therefore why even try? Well, because we are called to try. That's who we are as human beings and that's especially how we think of ourselves as Americans."

Later, Mrs Clinton defended the right to abortion and the Obama administration's decision to finance family planning overseas.

"I've been in hospitals in Brazil where half the women were enthusiastically and joyfully greeting new babies and the other half were fighting for their lives against botched abortions," she told Congress.

"I've been in African countries where 12 and 13-year-old girls are bearing children. I have been in Asian countries where the denial of family planning consigns women to lives of oppression and hardship.

"It is my strongly held view that you are entitled to advocate and everyone who agrees with you should be free to do so anywhere in the world, and so are we."