The talks resumed in Vienna on Tuesday and are expected to last through Wednesday, The New York Times reported.
Iran called on other negotiators, including the United States and Russia, to remained focused on progress to reach a permanent nuclear agreement.
"Important and tough discussions ahead today," Iran's foreign minister, Mohammad Javad Zarif, said on Twitter. "We have held our end of the bargain. Time for our counterparts to keep theirs."
Zarif was referring to Iran following a temporary nuclear accord that took effect on Jan. 20 in an effort to have temporary sanctions relief from the West.
Observers say the crisis in Ukraine may impact the talks, as Russia will be less inclined to work with other P5-plus-one countries -- the five permanent members of the United Nations Security Council, which include Russia, plus Germany. Weakened cooperation may reduce pressure on Iran to make concessions, experts said.
"I think that we all hope that the incredibly difficult situation in Ukraine will not create issues for this negotiation," said a senior American official, speaking on the condition of anonymity because of the sensitivity of the talks. "We hope that whatever happens in the days ahead, whatever actions we and the international community take, depending upon the decisions and the choices that Russia makes, that any actions that Russia subsequently takes will not put these negotiations at risk."
Gary Samore, a former senior aide on nonproliferation on the National Security Council in President Obama's first term, said: "If President Putin goes ahead with his apparent intention to annex Crimea, we're going to have to sanction Russia, and they are going to have to retaliate, and it's really going to screw up the P5-plus-one negotiations with Iran."