Iran can discuss other issues with the United States providing its 2015 nuclear deal with six powers is fully implemented, Iranian President Hassan Rouhani said on Thursday, adding that Tehran's missile capabilities were not negotiable.

The warning came as reports emerged that the European Union has cautioned Iran that it may withdraw from the nuclear deal if Tehran takes further steps away from the 2015 agreement.

Lift the sanctions, return to deal

Rouhani, speaking at a news conference in New York during the United Nations General Assembly, reiterated that fresh negotiations could not take place until preconditions were removed.

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"First, the deal should be fully implemented ... meaning that sanctions should be lifted and America should return to the nuclear deal ... and then other issues can be discussed as well," Rouhani said.

"We want America to remove its preconditions for talking to Iran, including its 'maximum pressure' policy against the Iranian nation."

Europeans issue warning

Iran has gradually scaled back its nuclear commitments since May, in retaliation for Washington's "maximum pressure" policy, but reports on Thursday indicated that the European signatories had privately warned that it may withdraw from the 2015 deal if Iran continues to move away from the pact.

Rouhani said Tehran's measures were reversible if the European parties to the deal carried out their promises to shield Iran's economy from U.S. penalties.

"The European parties have proved their inability or lack of determination ... to keep the deal alive in practice ... They want to save the pact, but they want Iran to pay the cost for it," Rouhani said.

Missiles non-negotiable

The U.S.-Iranian confrontation has ratcheted up since 2018, when U.S. President Donald Trump withdrew from the 2015 nuclear deal with major powers and reimposed sanctions that have crippled Iran's economy in a bid to force Tehran to renegotiate the pact.

Iran refuses to hold any talks with the United States unless Washington lifts sanctions on Tehran.

Trump wants to go beyond the deal to further curb Iran's nuclear program, halt its ballistic missile work and end its support for proxy forces in the Middle East. 

But Rouhani, stating Iran's official stance, said on Thursday Iran's defensive missile program was non-negotiable.

(With input from Reuters)