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Despite President Barack Obama’s objection, Republican and Democratic lawmakers in the United States will press ahead with a plan for more sanctions on Iran, the chairman of the Senate Foreign Relations Committee told Reuters on Wednesday.

Lawmakers, who say they fear Obama administration negotiators may not take a hard enough line with Tehran, are also at work on a separate bill to have Congress approve any final agreement on Iran's nuclear program, Senator Bob Corker, the chairman, told the news agency.

"There's continual efforts to try to figure out a way for Congress to play a role to strengthen whatever final deal may occur," the Tennessee Republican senator said.

Republican Senator Mark Kirk and Democratic Senator Robert Menendez are finalizing a bill for tougher sanctions on Iran if there is no final nuclear deal by June 30.

The bill, introduced by the two last year, was gaining momentum in Congress, but Obama lobbied hard against it and has more than once threatened to veto the bill if it passes.

The Senate Banking Committee is due to hold a hearing on Iran sanctions on Tuesday, said Corker, a member of that panel as well.

Kirk and Menendez’s bill did not come up for a vote in the Senate, then controlled by President Barack Obama's fellow Democrats. Now that the Senate is controlled by the Republicans, they have indicated that they plan to challenge Obama on Iran.

The White House has insisted passage of a sanctions bill now - even one that would impose new restrictions only if there is no deal by the deadline - could prompt Iran to back out of the nuclear talks with six world powers.

Although Republicans now hold a 54-46 seat majority in the Senate, Corker told Reuters he did not know if there would be enough votes - 67 - needed in the Senate to override an Obama veto of any Iran legislation.