Many US Republicans are unhappy with the weekend's deal with Iran to curb the country's controversial nuclear program in exchange for sanctions relief, and some experts said the Republican Party (GOP) may push for tougher measures.

"Congress is probably going to send up to (President Barack) Obama additional sanctions on Iran," Republican Strategist Ford O' Connell told Xinhua. "And the only question is whether the ( additional possible) sanctions occur before or after this interim deal."

After a preliminary deal on Iran's nuclear program was reached in Geneva early Sunday, the next six months will be crucial as the West monitors Tehran's progress in halting its controversial nuclear program, US experts said.

Experts and US officials said the agreement, which involves six world powers -- Britain, China, France, Russia, the United States and Germany (P5+1) -- represents the first step in curbing Iran's controversial nuclear program in exchange for easing the sanctions that have crippled the Islamic Republic's economy.

"If you are President Obama, the next six months with this deal is going to be tough, because you know it's not just Republicans griping. There's a lot of Democrats griping too," he said, adding that some Democrats will likely join GOP calls for stiffer sanctions.

Indeed, Democrat Senator Charles Schumer said Sunday he was " disappointed" with the deal, saying the terms did not seem " proportional".

Ed Royce, chairman of the US House Foreign Affairs Committee, on Saturday also expressed concerns. "I have serious concerns that this agreement does not meet the standards necessary to protect the United States and our allies," the Republican said in a statement.

"Instead of rolling back Iran's program, Tehran would be able to keep the key elements of its nuclear weapons-making capability. Yet we are the ones doing the dismantling - relieving Iran of the sanctions pressure built up over years," he said.

Republican Senator Marco Rubio Sunday also expressed opposition to the agreement, saying it "shows other rogue states that wish to go nuclear that you can obfuscate, cheat, and lie for a decade, and eventually the United States will tire and drop key demands."

O'Connell added that Obama is likely to keep the sanctions as limited as possible before the 2014 midterm Congressional elections in an effort not to arouse more attacks from Republicans.

Still, while Republicans are not completely happy with the deal, party members understand the talks are the first in more than three decades and do not want to nix the possibility of further negotiations and a permanent deal, despite their distrust of Tehran.

In the deal, Iran agreed to a six-month halt of its nuclear program while talks will continue to seal a final agreement and international observers monitor Iran's nuclear sites. But US officials have warned that any divergence by Iran will cause the U. S. to scrap the deal and impose harsher sanctions.

In return, the P5+1 is to provide limited, temporary, targeted and reversible sanctions relief, estimated at 7 billion US dollars, while maintaining the vast bulk of sanctions, including those placed on oil, finance and banking. Failure to meet those commitments will mean revoking sanctions relief, the White House said Saturday.

This deal marked the first significant talks between Iran and the US since the 1979 Iran hostage crisis, which kicked off three decades of sour relations. The next near-term steps will test whether Iran complies with its commitments.