There are some in Tehran convinced a final nuclear deal between Iran and world powers has already been sealed. Others are adamant there will never be a pact.

But as uncertainty persists over talks aimed at ending the 12-year standoff on Iran's disputed atomic programme, hurdles to an agreement are stacking up.

In Washington, lawmakers are considering imposing new sanctions on Tehran despite calls from the White House to give the negotiations more time.

Hardline Iranian MPs have responded with threats of their own and are drafting two bills that would undermine the talks.

With the going already tough -- the level of uranium enrichment Iran can conduct and a timetable for lifting sanctions are said to be blocking a deal -- analysts say pressure is being ratcheted up.

"Those who oppose any diplomacy between Iran and the West are already seeking to end the process," said Ellie Geranmayeh, an Iran specialist at the European Council on Foreign Relations.

"With time, they will gain further ammunition."

Although the June 30 deadline for a final agreement between Iran and the P5+1 powers -- Britain, China, France, Russia and the United States plus Germany -- is some way off, two earlier deadlines were missed.

In the Iranian capital, officials say the United States and other world powers need to show more flexibility in nailing down the hard details of a deal.

Iran's negotiators have given no indication a compromise is in the offing and supreme leader Ayatollah Ali Khamenei, who has the final word, last month voiced doubt that the US could be trusted.

- New sanctions will 'torpedo' talks -

Israel is lobbying hard against a final deal and Saudi Arabia is also wary, analysts say, and the negotiations could be scuttled if there is no definitive progress soon.

The biggest threat, they say, is the possibility of new US sanctions on Iran, which Foreign Minister Mohammad Javad Zarif has said would effectively "torpedo" the talks.

Such an outcome is likely the aim of Israeli Prime Minister Benjamin Netanyahu's address to the US Congress about Iran's nuclear efforts on March 3.

In a signal of White House disapproval at such a high-profile and untimely intervention -- an outline agreement with Iran is due on March 31 -- President Barack Obama will not meet Netanyahu.

Two US senators, Republican Mark Kirk and Democrat Robert Menendez, have said they will decide by March 24 whether to table new legislation.

If they can convince enough Democrats in the Republican-controlled Congress to back sanctions, they could have the super-majority needed to pass veto-proof legislation.

"(Menendez) will have an easier time collecting sponsors if there is no movement in the talks," Geranmayeh said.

In Tehran, MPs are considering two bills, one tearing up an interim deal that has reined in Tehran's nuclear activities and another allowing the Islamic republic's atomic scientists to speed up their operations.

- Time for a 'positive signal' -

Evidence of the souring atmosphere was clear when 21 hardline lawmakers recently condemned Zarif for taking a 15-minute stroll with US Secretary of State John Kerry during a break at negotiations in Geneva.

"Aversion to the deal has always had robust and noisy constituencies in Tehran and Washington, but both governments need to rise above the din if they want an agreement," said Siavush Randjbar-Daemi, a lecturer on Iran and the Middle East at Manchester University.

Kerry and Zarif may meet again this week in Munich and the two "need to thrash this out directly, hopefully through less controversial walks," Randjbar-Daemi said.

Amir Mohebbian, a Tehran-based analyst close to hardliners and several top figures, said the growing pressure could actually push the discussions forward.

But too much delay could cost President Hassan Rouhani -- who raised hopes of a return to prosperity via a nuclear deal -- vital public support.

When past deadlines were missed, Rouhani, a moderate in stark contrast to his hardline predecessor Mahmoud Ahmadinejad, went on state television to insist the talks were close to a result.

Iranians will now be looking for signs of more concrete progress, Mohebbian said.

"With the anniversary of the Islamic revolution on February 11, it would be a good time to send a positive signal to Iran's people."