GENEVA - The top diplomats from China, the United States, Russia, Britain, France and Germany gathered in Geneva on Saturday to join the ongoing international talks on the Iranian nuclear issue.

The talks between Iran and the United Nations Security Council's five permanent members plus Germany over its controversial nuclear program, scheduled to conclude on Friday, have extend into an unscheduled fourth day on Saturday.

After his arrival in Geneva, Chinese Foreign Minister Wang Yi held bilateral meetings in the afternoon with Russian Foreign Minister Sergei Lavrov, British Foreign Secretary William Hague and French Foreign Minister Laurent Fabius.

Wang pointed out that the talks have reached the crucial moment and relative parties should strive to push this round of talks forward to lay foundations for the political solution to the Iran nuclear issue.

The new round of talks starting from Wednesday is the third negotiations between Iran and the six countries in more than a month.

The last round of intensive talks, held from November 7 to 9, failed to nail down an interim deal as expected, but it was announced that a lot of concrete progress had been achieved while some differences remained.

A senior U.S. official said on condition of anonymity Wednesday evening that the focus of this round of talks is to get into details of "a possible first-step agreement, and the parameters of a comprehensive agreement and to see if we can narrow the remaining gaps necessary to conclude such an agreement."

Western countries have long been accusing Iran of developing nuclear weapons under the cover of a civilian nuclear program, but Iran insists that its nuclear program is for peaceful purposes.

Under the potential preliminary agreement, Iran would curb its nuclear program in exchange for modest sanction relief. The agreement, if successfully inked, would break up the decade-long deadlock over Tehran's disputed nuclear program.