The Obama administration's plans to complete efforts to restore diplomatic relations with Cuba in early July may run into trouble in Congress, where Republicans would have to approve funding to upgrade the U.S. Interests Section in Havana to a full-scale embassy, the Guardian reported.

Bruce Bagley, a Cuba expert at the University of Miami's department of international studies, told the British newspaper there might be little appetite within the GOP for the needed appropriations.

"Congress has to get on board," Bagley noted. But "I doubt sincerely we will see any big budgetary moves before President Obama leaves office, certainly not enough to open an entire Embassy. It's not clear it can happen unless Congress approves the money."

President Barack Obama is hoping to wrap up the historic agreement with Cuba next month and plans to dispatch Secretary of State John Kerry to Havana for a symbolic flag-raising ceremony, Reuters detailed. When exactly the secretary is supposed to make that trip, however, is uncertain because of Sec. Kerry's recovery from a broken leg and the looming deadline for a final nuclear deal with Iran.

The planned ceremony, along with the removal of Cuba from the U.S. list of countries that sponsor terrorism, can be viewed as "proof of progress in the conversation," Bagley admitted.

"This continued advancement and Kerry's trip have symbolic value to the (United States) in that they maintain the momentum, and other countries see that the (United States) wants to reestablish relations," Bagley said. But in Congress, "there is ferocious opposition from (Florida Sen.) Marco Rubio and others; change will be slow, whatever happens," the expert predicted.

Rubio, the son of Cuban immigrants and a Republican contender in the 2016 White House race, has said that Obama's decision to take the regime of President Raúl Castro off the terror list was "terrible" and sent "a chilling message to enemies that the White House is no longer serious about calling terrorism by its proper name."

The presidential candidate has been a leading proponent of legislation that would hinder negotiations by preventing Americans from spending on the island on businesses that would directly benefit the Cuban military.