In Berlin, Germany's foreign minister reported no progress in talks with Iran's chief nuclear negotiator ahead of the G-8 summit. And with the U.N. Security Council preparing to debate a third set of sanctions for Tehran's refusal to suspend enrichment, Britain raised the possibility of adding curbs on oil and gas investment to the limited measures against people and companies involved in Iran's nuclear and weapons programs.

"We advise them to give up stubbornness and childish games," Ahmadinejad said at a Tehran press conference. "Some say Iran is like a lion. It's seated quietly in a corner. We advise them not to play with the lion's tail."

"It is too late to stop the progress of Iran," Ahmadinejad said.

In Washington, U.S. State Department Spokesman Sean McCormack responded: "It isn't."

"He could make the decision today to take up the reasonable offer to negotiate with the rest of the world," McCormack said. "The international system is not going to be intimidated by these kinds of threats."

Adding to the tensions, shortly after Ahmadinejad's press conference Iran issued its harshest refutation yet of American demands to release dual citizens held in Iran on espionage charges.

Iran's Foreign Ministry said American abuses _ from prisoner mistreatment at Guantanamo to a UCLA police officer's shocking an Iranian-American student with a Taser _ showed Washington had no right to criticize Iran's human rights record.

"Instead of offering inefficient suggestions, America should assess its own tactics _ secret prisons, mistreatment and even inhuman treatment of prisoners in Guantanamo and Abu Ghraib," Foreign Ministry spokesman Mohammad Ali Hosseini said in a statement released by his office.

Iran has detained Haleh Esfandiari, director of the Middle East Program at the Washington-based Woodrow Wilson Center for Scholars; Kian Tajbakhsh with George Soros' Open Society Institute; journalist Parnaz Azima from the U.S.-funded Radio Farda; and Ali Shakeri, a peace activist and founding board member at the University of California, Irvine, Center for Citizen Peace building.

The Iranian statement referred to three Iranian-Americans charged with espionage and endangering national security, without providing names.

Hadi Ghaemi, an Iran expert for New York-based Human Rights Watch, said Esfandiari, 67, has been detained nearly a month without being seen by family or attorneys, raising concerns for her well-being.

"It is extremely, cruel and inhumane to treat an internationally respected scholar in such a way," he said. "It is not only the U.S. government that is criticizing this detention but people from across the political spectrum, from Noam Chomsky to major human rights organizations."

Ahmadinejad touched briefly on the issue at his press conference, saying Iran's judiciary would rule independently on the dual citizens' cases. He then returned to international issues including Iraq, fighting in Lebanon and Iran's nuclear ambitions.

Ahmadinejad's news conference was his first since talks between U.S. and Iranian diplomats in Baghdad on May 28 _ the first public talks between the two countries in nearly three decades. The two sides are supposed to meet again in less than a month.

The president scoffed at U.S. accusations that Iranian agents were helping fellow Shiite militants in Iraq but said that Tehran wanted to help calm the violence there.

"We are prepared, for the sake of the Iraqi people, to help," he said. "We won't spare any efforts."

The Security Council first imposed sanctions on Iran in December and modestly increased them in March over Iran's refusal to suspend enrichment. Iran says it is within its rights to pursue uranium enrichment for peaceful purposes.

The country's nuclear negotiator, Ali Larijani, said this week that the nuclear standoff could be settled in the coming weeks if the council drops preparations to debate the third round of sanctions.