By Arshad Mohammed and David Brunnstrom
WASHINGTON (Reuters) - U.S. negotiators can see a way to a very good nuclear agreement with Iran and are hopeful, but it remains to be seen whether a pact will be reached, a senior U.S. official said on Thursday.
The official, who spoke on condition of anonymity, said all seven nations involved in the talks were committed to a self-imposed June 30 deadline but it could slip "by a short bit" if necessary to get the substance of any agreement right.
U.S. Secretary of State John Kerry flies to Vienna on Friday to join the negotiations, which seek to reach an agreement under which Iran would curb its nuclear program in exchange for relief from economic sanctions that have crippled its economy.
In addition to Iran and the United States, Britain, China, France, Germany and Russia are all part of the talks.
The United States and some of its allies suspect that Iran is using its civil nuclear program as a cover to develop a nuclear weapons capability. Iran says its program is solely for peaceful purposes, such as making medical isotopes.
The two main sticking points in the talks are the timing and scope of sanctions relief and the monitoring and verification measures needed to ensure Iran does not cheat on an agreement.
"Despite these tough issues, we can truly see a path forward that gets us to a very good agreement here," the senior U.S. official told reporters during a conference call. "I am hopeful but it still remains to be seen whether we can get there."
Prominent former U.S. officials, including five with ties to President Barack Obama's first term, on Wednesday published a letter warning that a deal on curbing Iran's nuclear program was at risk of failing to provide adequate safeguards.
The letter's release ahead of what may be the finale to the nuclear talks appeared aimed at pressuring the White House to negotiate a stronger agreement.
Rather than undermining the U.S. negotiators, the letter could strengthen their hand with the Iranians by illustrating the challenge the White House may face in selling any agreement at home, including with the Republican-led U.S. Congress.
U.S. lawmakers themselves are sharpening warnings against a "weak" agreement and laying down red lines that, if crossed, could prompt Congress to trip up an agreement.
Several influential lawmakers said they do not want to see any sanctions lifted before Tehran begins complying with a deal, and they want a tough verification regime in which inspectors could visit Iranian facilities anytime and anywhere.
They also want Tehran to reveal past military dimensions of its nuclear program, particularly after Secretary of State Kerry seemed to soften the U.S. stance last week by saying Iran would not be pressed on this point.
Kerry telephoned Iran's foreign minister in recent days to tell him that Tehran must answer questions about whether its past atomic research was arms-related if it wants a nuclear deal, officials said.