By Louis Charbonneau

UNITED NATIONS (Reuters) - The United States and five other major powers are making progress in talks on a U.N. sanctions resolution against Iran, and Brazilian mediation is not slowing down the process, Washington's U.N. envoy said on Thursday.

As the United States, Britain, France, Germany, China and Russia press ahead with negotiations on the resolution, Brazilian President Luiz Inacio Lula da Silva heads to Tehran this weekend to help mediate a standoff over the Islamic Republic's nuclear program.

U.S. Ambassador to the United Nations Susan Rice was asked by reporters if the Brazilian mediation efforts were blocking progress on a fourth sanctions resolution against Iran to submit to the full Security Council.

"It is not impeding progress ... on reaching agreement at all," she said. "In fact, the P5-plus-one have been working intensively, regularly, both here and in capitals. I think we are making good progress."

The "P5-plus-one" refers to the five permanent veto-wielding members of the Security Council and Germany.

Rice said the mediation efforts were part of the two-track strategy the six powers are taking with Iran -- combining negotiations with the threat of more sanctions.

She added that progress on a sanctions resolution in New York would probably "strengthen Lula's hand" as he heads to Tehran.

Rice said she hoped Lula's message to Tehran would be that "pressure is mounting. Iran continues to have a choice. Assuming it continues to make the wrong choices, that pressure will intensify."

SANCTIONS NEGOTIATIONS

Brazil and Turkey are trying to revive a stalled U.N.-backed nuclear fuel swap proposal that Iran agreed to in October but later balked at. Under that deal, Iran would send a significant amount of enriched uranium to Russia and France for processing into fuel for a medical research reactor in Tehran.

Russia and China have urged Tehran to accept the deal, but Tehran has placed conditions on it that the Western powers find unacceptable.

Diplomats from the six powers met again in New York this week to discuss the draft resolution, U.N. envoys said. They said there were some disagreements on details in a U.S.-drafted sanctions proposal.

Russia and China, they said, dislike an arms embargo, a ban on new investments in Iran's energy sector and other proposed punitive measures in the draft and are urging the Americans and Europeans to soften them.

Western diplomats have varying estimates on when the six powers will have a draft resolution ready to present to the 15-nation Security Council for debate. They say they are still hoping for a vote on a sanctions resolution in June.

Several diplomats, all of whom spoke on condition of anonymity, said they did not expect a draft resolution to reach the council before Brazilian and Turkish mediation efforts have had a chance to succeed or fail.

Some said they expected the six to wait for the end of a major conference on the nuclear Non-Proliferation Treaty (NPT) that ends on May 28 before submitting a draft resolution to the council, while others said it might happen sooner.

Another complicating factor is the Lebanese presidency of the Security Council. Western diplomats have said that Lebanon, a nonpermament council member until the end of 2011, has made clear it would prefer not to have an Iran sanctions resolution reach the council during its presidency this month.

The Iranian-backed militant group Hezbollah is in the Lebanese government, which means Lebanon will most likely not vote in favor of new sanctions against Iran, diplomats say.

(Editing by Xavier Briand)