US President Barack Obama said Monday there was no reason to extend nuclear talks with Iran once again, stressing the question now is whether Tehran truly wants an agreement.

"I don't see a further extension being useful if they have not agreed to the basic formulation and the bottom line that the world requires to have confidence that they're not pursuing a nuclear weapon," Obama said at a joint press conference with visiting German Chancellor Angela Merkel.

Obama said the issues standing in the way of a comprehensive agreement were no longer technical.

"The issues now are -- does Iran have the political will and the desire to get a deal done?" he said.

His comments followed a White House meeting with Merkel, whose country along with the United States is a member of the P5+1 group negotiating with Tehran.

Two deadlines for reaching a permanent agreement on Iran's nuclear program have passed, and skepticism about Tehran's intentions have been growing.

Under an interim agreement, Iran has diluted its stock of fissile materials from 20 percent enriched uranium to five percent in exchange for limited sanctions relief.

But negotiators now must reach a political consensus by March 31 and then a final deal setting out the agreement in technical detail by June 30.

Meanwhile, pressure is growing in the US Congress for spelling out ahead of time the sanctions Iran would face if there is no deal, which the US administration vehemently opposes.

Republicans have further angered the administration by inviting Israel's Prime Minister Benjamin Netanyahu to address a joint meeting of Congress early next month to make his case against a deal.

Obama acknowledged "a very real difference" with Netanyahu, and reiterated his opposition to the sanctions moves by Congress.

While he said the United States and Israel have an "unbreakable bond," Obama warned that preserving meant make sure "it doesn't get clouded with what could be perceived as partisan politics."

Obama said he and other allies, including Merkel and Britain's Prime Minister David Cameron, agree "that it does not make sense to sour the negotiations a month or two before they're about to be completed."

"And as I've said to Congress, I'll be the first to work with them to apply stronger measures against Iran. But what's the rush?" he said.

He noted that if the negotiations fail to produce an agreement, the "options are narrow and they're not attractive."

The United States and Germany are joined by Britain, China, France and Russia -- the so-called P5+1 -- in the negotiations with Tehran.

US Secretary of State John Kerry met Sunday in Germany with his Iranian counterpart Mohammad Javad Zarif to step up efforts to reach an agreement.

But Ayatollah Ali Khamenei, Iran's supreme leader, was quoted as saying he agreed with the Americans that it was better to have no deal than a bad deal.

"It's better to have no agreement than one that goes against our national interests," he said.