WASHINGTON (AFP) - US Secretary of State Hillary Clinton said Wednesday the Obama administration is prepared to push for tough sanctions against Iran if the new dialogue fails.

In her first testimony to Congress since her confirmation as the US chief diplomat in January, Clinton also defended President Barack Obama's overtures to communist Cuba, a regime she said is ending, and voiced fears over Taliban advances in Pakistan.

Clinton sought to reassure Howard Berman, chairman of the House Foreign Affairs Committee, who backed dialogue with Tehran but asked if Washington could garner enough support to impose "crippling sanctions" if it fails.

She said the Obama administration had "more leverage with other nations" through its role in negotiations with the other four UN Security Council members -- Russia, China, France and Britain -- plus Germany.

Its additional decision to invite Iran "as I did, to the conference in The Hague on Afghanistan increases even further our ability to ask more from other nations," she added.

Iran must reciprocate or face the consequences, she said during a hearing in which Democrats showered her with praise for boosting the image of Washington abroad after eight years under Republican president George W. Bush.

"But we are also laying the groundwork for the kind of very tough, I think you said crippling, sanctions that might be necessary in the event that our offers are either rejected or the process is inconclusive or unsuccessful," she told Berman.

The UN Security Council has already imposed three rounds of sanctions on Iran, though they are not as strong as Washington wants amid resistance from Moscow and Beijing.

Iran said on Wednesday it is ready for "constructive dialogue" with world powers on its nuclear drive, after President Mahmoud Ahmadinejad welcomed the US policy shift towards Tehran.

But Iran refuses to halt its nuclear program, which Western nations fear could be a cover for efforts to build an atomic bomb, despite Tehran's insistence that it is aimed only at generating electricity.

Clinton also said Taliban advances pose "an existential threat" to Pakistan and urged Pakistanis worldwide to speak out against a government policy ceding ground to militants bent on overthrowing the nuclear-armed state of Pakistan.

Pakistani officials said Wednesday that Taliban militants in Pakistan's Swat valley have moved closer to Islamabad in a bid to broaden their control despite a deal designed to allow sharia law to end extremist violence.

Indications of a spread of Taliban activism will likely fuel criticism of the accord for Swat agreed by Pakistani President Asif Ali Zardari earlier this month, which the United States said amounted to capitulation.

Clinton added later that the Pakistan government is "basically abdicating to the Taliban and the extremists."

On the Middle East, Clinton defended the administration's push for a two-state solution for Israelis and Palestinians, after one lawmaker suggested Palestinian Authority president Mahmud Abbas was too weak to lead a state.

Clinton defended US efforts to improve ties with Cuba as the country faces a transition.

"You can see there is beginning to be a debate," Clinton said, referring to differences between President Raul Castro and his older brother, Fidel, the ailing longtime leader, over Obama's overture.

"I mean this is a regime that is ending. It will end at some point," Clinton said.

The secretary of state also called for a new approach to President Hugo Chavez's Venezuela, which has been a thorn in the US side, after saying previous attempts to isolate him had not worked.

At times Clinton slapped down Republican lawmakers who took the administration to task over its policy to engage former foes and abandon the harsh interrogations of terror suspects.

"It won't surprise you that I don't consider him a particularly reliable source," she said, dismissing former Vice President Dick Cheney's claim that classified documents prove that such interrogations yielded key intelligence.