Iran’s foreign minister said on Thursday “significant progress” has been made in nuclear talks with the six major powers but more discussions were needed to reach an agreement over how to resolve a 12-year standoff over Tehran’s nuclear work.

“We have made significant progress in the talks but still we have not agreed on the reviewed solutions,” Mohammad Javad Zarif told reporters after eight days of talks in the Swiss city of Lausanne.

“We are working on setting parameters of the issues that will led to drafting the final deal by end of June.”¬†

Meanwhile, influential Republican senators expressed concern that extended nuclear talks with Iran won’t yield a beneficial agreement.

Arizona Senator John McCain told a town hall meeting in Denver that he did not trust US Secretary of State John Kerry to secure an agreement that would protect the US and Israel.

“Doesn’t it look a little unseemly when the greatest nation in the world, their Secretary of State keeps begging for another day of negotiations? Doesn’t that give the advantage, at least psychologically to the Iranians?” McCain said.

When asked by the forum moderator if he trusted Secretary of State John Kerry to broker a deal that would protect the U.S. and Israel, McCain answered simply “No.”

Major powers and Iran negotiated into the early hours of Thursday (April 2) on Tehran’s nuclear program two days past their deadline, with diplomats saying prospects for a preliminary agreement were finely balanced between success and collapse.

Senator Lindsay Graham was also skeptical that a fruitful nuclear deal could be reached.

“Here’s what I’m afraid of: that we’re going to get a deal that will be in the eyes of the Israelis and the Arabs, a unacceptable deal” Graham said.

The negotiations taking place in Lausanne, Switzerland are aimed at blocking Iran’s capacity to build a nuclear bomb in exchange for lifting sanctions, have become bogged down over crucial details of the accord, even as the broad outlines of an agreement have been reached.

After negotiators passed an original self-imposed deadline of midnight on Tuesday, they remained locked in talks through to the early hours of Thursday.

Kerry and German Foreign Minister Frank-Walter Steinmeier said they would stay at least until Thursday in an effort to seal the “political” agreement, a milestone towards a final pact due by the end of June.