Spoilers and "dark forces" are attempting to wreck efforts to clinch a historic compromise between Iran and the west on the country's nuclear programme, senior members of Iran's negotiating team have told the Guardian.

Speaking before a new round of expert-level talks, due to begin on Tuesday in New York, Seyed Abbas Araqchi, the deputy foreign minister for legal and international affairs, said Iran remained hopeful that a comprehensive agreement could be reached by the 20 July deadline.

But Araqchi, part of Iran's three-man lead negotiating team, warned that many pitfalls remained, including a chronic lack of trust between the US and Iran, a host of inter-related technical issues, and outside attempts to derail the process.

"There are spoilers everywhere who don't want an agreement, there are dark forces who don't like this process … It is clear some people don't want to resolve this issue in a peaceful and logical way," Araqchi said during an exclusive interview at the foreign ministry in Tehran.

"I don't want to use the word 'warmongers'. But these people want continuing tension, a continuing crisis in our region. They don't want the sanctions on Iran to end. They don't want Iran to be a major player in this region, although in fact it already is."

Araqchi did not name any country but his remarks appeared aimed at the Israeli government, which believes Iran is intent on covertly developing nuclear weapons – a claim Tehran firmly denies. Israel's prime minister, Binyamin Netanyahu, has been highly critical of the talks between Iran and the P5+1, the five permanent UN security council members plus Germany.

Araqchi noted that rightwing Republicans in the US Congress had opposed an interim agreement reached in Geneva last November that afforded Iran limited sanctions relief in return for slowing its nuclear programme. Congress must approve any final deal.

But he conceded that he and the other negotiators were under fire within Iran, where hardliners have condemned earlier concessions. "Obviously in Iran we have a pluralistic society. There are different ideas, different opinions, there are those who believe we should not negotiate at all.

"There are some people in the Majlis [parliament] who are very critical and we must answer to them. We should let all the voices be heard."

Araqchi said there was considerable pressure on the Iranian team to get an acceptable result. Although the atmosphere in the talks was friendly and constructive, carrying responsibility for the negotiations was "very stressful".

The two other team members are Mohammad Javad Zarif, Iran's foreign minister, and Majid Takht-Ravanchi, the deputy foreign minister for European and American affairs.

"We feel supported by the majority of the society. The political structure of the country supports us as a whole. That does not mean there is not criticism."

Araqchi said the talks were going in the right direction. "Whether it gets to a conclusion is something else. Obviously we are hopeful. For our part, we are very serious and we have goodwill. If the other side reciprocates, hopefully we will come to an end. But anything can happen."

He said the next top-level round of talks, due to begin in Vienna on 13 May, would be the most difficult part so far, because the parties had agreed to start writing a draft of a final agreement.

The many outstanding, highly complex technical issues were all linked, he said. The P5+1 countries did not always have a united position. And it was understood that nothing was agreed until everything was agreed. "We could get 95% agreement and the last 5% could ruin everything."

Araqchi said Iran's centrist president, Hassan Rouhani, was keen for a deal in order to end sanctions but the supreme leader, Ali Khamenei, a religious conservative, was "not optimistic" agreement could be reached.

"Rouhani has invested a lot in this, he has raised expectations … But I think people understand the complexities of the situation. I don't think it would be a big blow to Rouhani if there is no agreement. People understand he has done his best."

Iranian insiders say that while Araqchi has the ear of the supreme leader and Zarif is Rouhani's right-hand man, Takht-Ravanchi, the third member of the team, is the brains behind the Iranian operation.

Speaking to the Guardian in his office in Tehran, Takht-Ravanchi said: "I am not pessimistic regarding a deal. We are doing our best to finish the job before 20 July." Extending the deadline was not ruled out, he said.

The Iranian government had to reassure domestic critics that a deal was in the nation's interests. For its part the Obama administration must ensure any agreement was fully implemented, including obtaining congressional approval.

"All the sanctions must be lifted if we are to have an agreement," he said. "It is up to them [the Americans and Europeans] to do their homework."

Ravanchi said apparent US attempts to introduce what Iran regards as irrelevant issues into the negotiations, such as human rights and Iran's long-range missile capabilities, could not be allowed and would not be discussed.

Asked whether the Ukraine crisis had strengthened Iran's hand by highlighting Europe's need for non-Russian oil and gas supplies, as some in Tehran believe, Ravanchi said the negotiators were focused solely on the nuclear issue.

"We have nothing else in mind. Naturally Iran and Europe could have much better cooperation on the economy, trade, energy. We believe there is much room for improvement."

Enmity between the US and Iran dating back to the 1979 revolution constantly overshadowed the talks and was hard to escape, Araqchi said. "We have tried to rebuild confidence but we have not been very successful … What we need now is wisdom and luck."