Brazilian President Luiz Inacio Lula da Silva's upcoming visit to Tehran may be Iran's last chance to engage the world over its nuclear program before new UN sanctions are imposed, a US official said Thursday.

The remarks came as Obama administration officials signaled they were making "good progress" toward enlisting support for sanctions from not only Russia but also China, both of which are veto-wielding UN Security Council members.

China has been the main holdout to a fourth round of sanctions on the UN Security Council, along with Brazil, Turkey and Lebanon. Russia over the last few months has expressed greater openness to sanctions.

"I think we would view the Lula visit as perhaps the last big shot at engagement," a senior State Department official told reporters on the condition of anonymity.

Lula, who has backed a diplomatic solution to the nuclear standoff, is scheduled to visit Iran on Sunday and Monday.

The US official could not rule out a high-level Turkish official visiting Tehran for Iran-Brazil-Turkey talks.

If Iran does not change tack after Lula's visit, the official said, "that should communicate something of great significance and countries like Brazil, Turkey and others on the council should very definitely draw conclusions from that."

The United States has insisted for weeks it is keeping the door open to talks with Iran but says it is increasingly skeptical that the Islamic republic will engage the world community without some additional pressure.

The message was underlined again in a telephone call US Secretary of State Hillary Clinton had Thursday with Turkish Foreign Minister Ahmet Davutoglu.

Clinton told him that "Iran's recent diplomacy was an intent to stop Security Council action without" addressing international concerns about its nuclear program, her spokesman Philip Crowley said.

President Barack Obama's administration, for example, complains that Iran has failed to respond to a confidence-building offer made way back last October by the International Atomic Energy Agency (IAEA), the UN nuclear watchdog.

The offer would allow Iran to ship most of its low-grade uranium abroad so that it could be further enriched and returned to Iran to make medical isotopes at the Tehran Research Reactor.

The United States is trying to get Iran to stop enriching uranium, which can be used as fuel either for civilian power reactors or atomic weapons.

In New York, Susan Rice, the US ambassador to the United Nations, said Lula's visit to Iran was "not impeding progress... on reaching agreement at all" among the P5-plus-1.

Rice was referring to the permanent five UN Security Council members -- the United States, Russia, China, Britain and France -- plus Germany, which is the group that has for years been leading efforts to curb Iran's nuclear ambitions.

"In fact, the P5-plus-1 have been working intensively, regularly, both here and in capitals," she said.

"We're making good progress, and we have always maintained that this is dual track policy with pressure and the opportunity for diplomatic engagement," said Rice.

"And I think the progress that the P5 is making in fact, perhaps, strengthens President Lula's hand," when he visits Tehran, Rice said.

Crowley said earlier that Clinton and China's state councilor Dai Bingguo acknowledged "good progress" had been made when they talked over the phone Wednesday about efforts to impose new sanctions on Iran.

Obama and his Russian counterpart Dmitry Medvedev noted in a phone call Thursday the "good progress" being made on Iran sanctions and "agreed to instruct their negotiators to intensify their efforts to reach conclusion as soon as possible," the White House said.

There were no immediate remarks from either the Chinese or the Russians.

Crowley said: "In the next few weeks, we expect to table a resolution in New York."