UNITED NATIONS (AFP) - United States (US) leadership at the United Nations is indispensable, Washington's new envoy to the global body said on Monday.

Ms Samantha Power presented her credentials to UN Secretary General Ban Ki Moon following her confirmation by the US Senate late last week.

The 42-year-old replaces Susan Rice whom President Barack Obama named in June to be his national security advisor.

"As I made clear in my testimony before the Senate, the UN is critical to a range of US interests and US leadership at the UN is indispensable to making progress on those interests," Ms Power said before meeting with Ban.

Some of the issues Washington and the UN must work together on include terrorism, "mass atrocities in Syria," as well as South Sudan, global poverty and the crackdown on civil society around the world, she said.

Ms Power, a Harvard Law School graduate and former special assistant to Mr Obama, enjoyed widespread support during the confirmation process as someone who would be an effective and eloquent advocate for US interests.