The new US ambassador made her UN debut Monday, pledging "vigorous" and "direct" nuclear diplomacy with Iran but warning of increased pressure if Tehran refuses to halt uranium enrichment.

Susan Rice told reporters that President Barack Obama's administration looked forward to "engaging in vigorous diplomacy that includes direct diplomacy with Iran, as well as continued collaboration and partnership with" the five permanent members of the UN Security Council and Germany.

"We will look at what is necessary and appropriate with respect to maintaining pressure toward that goal of ending Iran's nuclear program," she added.

"Dialogue and diplomacy must go hand in hand with a very firm message from the United States and the international community that Iran needs to meet its obligations as defined by the Security Council and its continued refusal to do so will only cause pressure to increase," the 44-year-old Rice said.

At the White House, spokesman Robert Gibbs said the new US envoy to the UN merely restated Obama's view that his administration would use "all elements of our national power to address the concerns that we have with Iran."

The five permanent members of the Security Council -- Britain, China, France, Russia and the United States -- plus Germany have offered Tehran a set of economic and energy incentives in exchange for halting its uranium enrichment program which the West sees as a cover to acquire a nuclear weapons capability.

But Tehran is pressing on with its sensitive nuclear fuel work, insisting that its nuclear program is peaceful and solely geared toward electricity generation.

The Security Council has already adopted four resolutions -- three of which included sanctions -- requiring Iran to suspend uranium enrichment.

Rice also said her 45-minute chat with Un chief Ban Ki-moon covered a wide range of other issues, including climate change, the poverty-reduction Millennium Development Goals, UN peacekeeping, the Middle East and Darfur.

On the recent Gaza conflict between Israel and the Palestinian Islamist movement Hamas, she said that "what is required is a durable ceasefire."

"We will work diplomatically and through other means to try to support efforts to ensure that that ceasefire is lasting, and in that context for border crossings (into Gaza) to open and be available for humanitarian as well as day-to-day economic development imperatives," she added.

She recalled Obama's vow to push for "the longer-term effort to promote a peaceful resolution of the conflict based on two states living side by side in peace and security."

On Darfur, where Sudanese troops clashed with rebels earlier Monday, Rice said Washington remained "very deeply concerned about the ongoing genocide."

"The priority at this point has to be effective protection for civilians," added Rice, noting that she discussed with Ban prospects for "effective efforts to support the full and complete deployment of UNAMID (the UN-African Union mission in Darfur)."

She reiterated that the Obama administration was committed to a strong partnership with the UN, an institution often criticized by some US lawmakers over allegations of corruption and mismanagement.

"President Obama's view is clear, that our security and well-being can best be advanced in cooperation and in partnership with other nations. And there is no more important forum, for that effective cooperation, than the United Nations," said Rice. "I will listen. I will engage."

But she stressed that the world body "still has room for great improvement" and said she and Ban touched on "the challenge of how we make the organization more efficient and effective" through continued management reform.

Rice is the first black American woman to serve in the UN post. Unlike her predecessor, Afghan-born Zalmay Khalilzad, she has cabinet rank.

Rice, a seasoned, tough-minded Africa expert, was a top foreign policy adviser to the Obama election campaign.

She previously served on the National Security Council and as assistant secretary of state for African affairs under president Bill Clinton in the 1990s.