Iran’s ambassador to the United Nations said Friday that negotiations with the big powers over the disputed Iranian nuclear program had reached a “critical point,” implicitly raising the possibility of failure, because of the new regimen of harsh economic sanctions that have just been imposed on his country by the United States and European Union.

“This by itself indicates that they are not willing to engage with us in a meaningful dialogue,” the ambassador, Mohammad Khazaee, told reporters at a news conference convened by Iran’s United Nations mission to present its view of the nuclear negotiations, which have made scant progress after three rounds of high-level talks since April.

A meeting of lower-level negotiators representing Iran and the so-called P5-plus-1 group of nations — the five permanent members of the United Nations Security Council plus Germany — is set for Tuesday in Istanbul.

Mr. Khazaee rarely holds news conferences, and the timing suggested that Iranian officials were posturing to lay the groundwork for possible disappointment or worse at the meeting in Turkey.